diff --git a/README.md b/README.md
index f88f61380..a9760f60b 100644
--- a/README.md
+++ b/README.md
@@ -25,7 +25,7 @@ which aims to provide many conveniences over raw assembly code (even when using
 - breakpoints, that let the Vice emulator drop into the monitor if execution hits them
 - source code labels automatically loaded in Vice emulator so it can show them in disassembly
 - conditional gotos
-- some code optimizations (such as not repeatedly loading the same value in a register) 
+- various code optimizations (code structure, logical and numerical expressions, ...) 
 - @todo: loops
 - @todo: memory block operations
 
diff --git a/il65/compile.py b/il65/compile.py
index 9d2fcb618..3e07b2c86 100644
--- a/il65/compile.py
+++ b/il65/compile.py
@@ -11,10 +11,7 @@ import sys
 import linecache
 from typing import Optional, Tuple, Set, Dict, List, Any, no_type_check
 import attr
-from .plyparse import parse_file, ParseError, Module, Directive, Block, Subroutine, Scope, VarDef, LiteralValue, \
-    SubCall, Goto, Return, Assignment, InlineAssembly, Register, Expression, ProgramFormat, ZpOptions,\
-    SymbolName, Dereference, AddressOf, IncrDecr, AstNode, datatype_of, coerce_constant_value, \
-    check_symbol_definition, UndefinedSymbolError, process_expression, AugAssignment
+from .plyparse import *
 from .plylex import SourceRef, print_bold
 from .datatypes import DataType, VarType
 
@@ -38,7 +35,7 @@ class PlyParser:
             self.check_all_symbolnames(module)
             self.create_multiassigns(module)
             self.check_and_merge_zeropages(module)
-            self.process_all_expressions(module)
+            self.simplify_some_assignments(module)
             if not self.imported_module:
                 # the following shall only be done on the main module after all imports have been done:
                 self.apply_directive_options(module)
@@ -75,7 +72,14 @@ class PlyParser:
         # perform semantic analysis / checks on the syntactic parse tree we have so far
         # (note: symbol names have already been checked to exist when we start this)
         previous_stmt = None
+        encountered_blocks = set()  # type: Set[Block]
         for node in module.all_nodes():
+            if isinstance(node, Block):
+                parentname = (node.parent.name + ".") if node.parent else ""
+                blockname = parentname + node.name
+                if blockname in encountered_blocks:
+                    raise ValueError("block names not unique:", blockname)
+                encountered_blocks.add(blockname)
             if isinstance(node, Scope):
                 if node.nodes and isinstance(node.parent, (Block, Subroutine)):
                     if isinstance(node.parent, Block) and node.parent.name != "ZP":
@@ -170,27 +174,11 @@ class PlyParser:
         for node in module.all_nodes(SymbolName):
             check_symbol_definition(node.name, node.my_scope(), node.sourceref)     # type: ignore
 
-    def process_all_expressions(self, module: Module) -> None:
-        # process/simplify all expressions (constant folding etc)
-        encountered_blocks = set()    # type: Set[Block]
+    def simplify_some_assignments(self, module: Module) -> None:
+        # simplify some assignment statements,
+        # note taht most of the expression optimization (constant folding etc) is done in the optimizer.
         for node in module.all_nodes():
-            if isinstance(node, Block):
-                parentname = (node.parent.name + ".") if node.parent else ""
-                blockname = parentname + node.name
-                if blockname in encountered_blocks:
-                    raise ValueError("block names not unique:", blockname)
-                encountered_blocks.add(blockname)
-            elif isinstance(node, Expression):
-                try:
-                    evaluated = process_expression(node, node.sourceref)
-                    if evaluated is not node:
-                        # replace the node with the newly evaluated result
-                        node.parent.replace_node(node, evaluated)
-                except ParseError:
-                    raise
-                except Exception as x:
-                    self.handle_internal_error(x, "process_expressions of node {}".format(node))
-            elif isinstance(node, IncrDecr) and node.howmuch not in (0, 1):
+            if isinstance(node, IncrDecr) and node.howmuch not in (0, 1):
                 _, node.howmuch = coerce_constant_value(datatype_of(node.target, node.my_scope()), node.howmuch, node.sourceref)
                 attr.validate(node)
             elif isinstance(node, VarDef):
@@ -485,17 +473,6 @@ class PlyParser:
             print("\x1b[0m", file=out, end="", flush=True)
         raise exc   # XXX temporary to see where the error occurred
 
-    def handle_internal_error(self, exc: Exception, msg: str="") -> None:
-        out = sys.stdout
-        if out.isatty():
-            print("\x1b[1m", file=out)
-        print("\nERROR: internal parser error: ", exc, file=out)
-        if msg:
-            print("    Message:", msg, end="\n\n")
-        if out.isatty():
-            print("\x1b[0m", file=out, end="", flush=True)
-        raise exc
-
 
 class Zeropage:
     SCRATCH_B1 = 0x02
diff --git a/il65/datatypes.py b/il65/datatypes.py
index eaf8e33f7..cc75757fc 100644
--- a/il65/datatypes.py
+++ b/il65/datatypes.py
@@ -5,11 +5,8 @@ Here are the data type definitions and -conversions.
 Written by Irmen de Jong (irmen@razorvine.net) - license: GNU GPL 3.0
 """
 
-import math
 import enum
-from typing import Tuple, Union
 from functools import total_ordering
-from .plylex import print_warning, SourceRef
 
 
 @total_ordering
diff --git a/il65/emit/incrdecr.py b/il65/emit/incrdecr.py
index 619e53c36..59fd626ef 100644
--- a/il65/emit/incrdecr.py
+++ b/il65/emit/incrdecr.py
@@ -193,11 +193,13 @@ def generate_incrdecr(out: Callable, stmt: IncrDecr, scope: Scope) -> None:
                         out("\vjsr  c64flt.float_add_one")
                     else:
                         out("\vjsr  c64flt.float_sub_one")
-            elif NOTYETIMPLEMENTED:     # XXX  for the  float += otherfloat cases
+            else:
+                # XXX  for the  float += otherfloat cases
+                print("FLOAT INCR/DECR BY", stmt.howmuch)  # XXX
                 with preserving_registers({'A', 'X', 'Y'}, scope, out, loads_a_within=True):
-                    out("\vlda  #<" + stmt.value.name)
+                    # XXX out("\vlda  #<" + stmt.value.name)
                     out("\vsta  c64.SCRATCH_ZPWORD1")
-                    out("\vlda  #>" + stmt.value.name)
+                    # XXX out("\vlda  #>" + stmt.value.name)
                     out("\vsta  c64.SCRATCH_ZPWORD1+1")
                     out("\vldx  #<" + what_str)
                     out("\vldy  #>" + what_str)
@@ -205,8 +207,6 @@ def generate_incrdecr(out: Callable, stmt: IncrDecr, scope: Scope) -> None:
                         out("\vjsr  c64flt.float_add_SW1_to_XY")
                     else:
                         out("\vjsr  c64flt.float_sub_SW1_from_XY")
-            else:
-                raise CodeError("incr/decr missing float constant definition")
         else:
             raise CodeError("cannot in/decrement memory of type " + str(target.datatype), stmt.howmuch)
 
diff --git a/il65/emit/variables.py b/il65/emit/variables.py
index d42295f1d..02b4d799a 100644
--- a/il65/emit/variables.py
+++ b/il65/emit/variables.py
@@ -129,6 +129,7 @@ def generate_block_vars(out: Callable, block: Block, zeropage: bool=False) -> No
             _generate_string_var(out, vardef)
         else:
             raise CodeError("invalid const type", vardef)
+    # @todo float constants that are used in expressions
     out("; memory mapped variables")
     for vardef in vars_by_vartype.get(VarType.MEMORY, []):
         # create a definition for variables at a specific place in memory (memory-mapped)
diff --git a/il65/main.py b/il65/main.py
index 38b5ab597..d730ac526 100644
--- a/il65/main.py
+++ b/il65/main.py
@@ -83,7 +83,7 @@ def main() -> None:
     parsed_module = parser.parse_file(args.sourcefile)
     if parsed_module:
         if args.nooptimize:
-            print_bold("not optimizing the parse tree!")
+            print_bold("Optimizations disabled!")
         else:
             print("\nOptimizing code.")
             optimize(parsed_module)
diff --git a/il65/optimize.py b/il65/optimize.py
index 2bb121955..bccd8472d 100644
--- a/il65/optimize.py
+++ b/il65/optimize.py
@@ -6,21 +6,34 @@ eliminates statements that have no effect, optimizes calculations etc.
 Written by Irmen de Jong (irmen@razorvine.net) - license: GNU GPL 3.0
 """
 
-from typing import List, no_type_check, Union
-from .plyparse import AstNode, Module, Subroutine, Block, Directive, Assignment, AugAssignment, Goto, Expression, IncrDecr,\
-    datatype_of, coerce_constant_value, AssignmentTargets, LiteralValue, Scope, Register, SymbolName, \
-    Dereference, TargetRegisters, VarDef
+import sys
+from typing import List, no_type_check, Union, Any
+from .plyparse import *
 from .plylex import print_warning, print_bold, SourceRef
-from .datatypes import DataType
+from .datatypes import DataType, VarType
 
 
 class Optimizer:
     def __init__(self, mod: Module) -> None:
         self.num_warnings = 0
         self.module = mod
+        self.optimizations_performed = False
 
     def optimize(self) -> None:
         self.num_warnings = 0
+        self.optimizations_performed = True
+        # keep optimizing as long as there were changes made
+        while self.optimizations_performed:
+            self.optimizations_performed = False
+            self._optimize()
+        # remaining optimizations that have to be done just once:
+        self.remove_unused_subroutines()
+        self.remove_empty_blocks()
+
+    def _optimize(self) -> None:
+        self.constant_folding()
+        # @todo expression optimization: reduce expression nesting
+        # @todo expression optimization: simplify logical expression when a term makes it always true or false
         self.create_aug_assignments()
         self.optimize_assignments()
         self.remove_superfluous_assignments()
@@ -28,11 +41,32 @@ class Optimizer:
         self.optimize_multiassigns()
         # @todo optimize some simple multiplications into shifts  (A*=8 -> A<<3)
         # @todo optimize addition with self into shift 1  (A+=A -> A<<=1)
-        self.remove_unused_subroutines()
         self.optimize_goto_compare_with_zero()
         self.join_incrdecrs()
         # @todo analyse for unreachable code and remove that (f.i. code after goto or return that has no label so can never be jumped to)
-        self.remove_empty_blocks()
+
+    def handle_internal_error(self, exc: Exception, msg: str="") -> None:
+        out = sys.stdout
+        if out.isatty():
+            print("\x1b[1m", file=out)
+        print("\nERROR: internal parser/optimizer error: ", exc, file=out)
+        if msg:
+            print("    Message:", msg, end="\n\n")
+        if out.isatty():
+            print("\x1b[0m", file=out, end="", flush=True)
+        raise exc
+
+    def constant_folding(self) -> None:
+        for expression in self.module.all_nodes(Expression):
+            try:
+                evaluated = process_expression(expression, expression.sourceref)    # type: ignore
+                if evaluated is not expression:
+                    # replace the node with the newly evaluated result
+                    expression.parent.replace_node(expression, evaluated)
+            except ParseError:
+                raise
+            except Exception as x:
+                self.handle_internal_error(x, "process_expressions of node {}".format(expression))
 
     def join_incrdecrs(self) -> None:
         for scope in self.module.all_nodes(Scope):
@@ -88,6 +122,7 @@ class Optimizer:
                             incrdecr = self._make_incrdecr(incrdecrs[0], target, total, "--")
                             scope.replace_node(incrdecrs[0], incrdecr)
                     if replaced:
+                        self.optimizations_performed = True
                         self.num_warnings += 1
                         print_warning("{}: merged a sequence of incr/decrs or augmented assignments".format(incrdecrs[0].sourceref))
                 incrdecrs.clear()
@@ -122,6 +157,7 @@ class Optimizer:
                     operator = expr.operator + '='
                     aug_assign = self._make_aug_assign(assignment, assignment.left.nodes[0], num_val, operator)
                     assignment.my_scope().replace_node(assignment, aug_assign)
+                    self.optimizations_performed = True
                 continue
             if expr.operator not in ('+', '*', '|', '^'):  # associative operators
                 continue
@@ -130,11 +166,13 @@ class Optimizer:
                 operator = expr.operator + '='
                 aug_assign = self._make_aug_assign(assignment, assignment.left.nodes[0], num_val, operator)
                 assignment.my_scope().replace_node(assignment, aug_assign)
+                self.optimizations_performed = True
             elif isinstance(expr.left, (LiteralValue, SymbolName)) and self._same_target(assignment.left.nodes[0], expr.right):
                 num_val = expr.left.const_num_val()
                 operator = expr.operator + '='
                 aug_assign = self._make_aug_assign(assignment, assignment.left.nodes[0], num_val, operator)
                 assignment.my_scope().replace_node(assignment, aug_assign)
+                self.optimizations_performed = True
 
     def remove_superfluous_assignments(self) -> None:
         # remove consecutive assignment statements to the same target, only keep the last value (only if its a constant!)
@@ -146,6 +184,7 @@ class Optimizer:
                     if isinstance(node.right, (LiteralValue, Register)) and node.left.same_targets(prev_node.left):
                         if not node.left.has_memvalue():
                             scope.remove_node(prev_node)
+                            self.optimizations_performed = True
                             self.num_warnings += 1
                             print_warning("{}: removed superfluous assignment".format(prev_node.sourceref))
                 prev_node = node
@@ -160,6 +199,7 @@ class Optimizer:
             if isinstance(assignment, Assignment):
                 if all(lv == assignment.right for lv in assignment.left.nodes):
                     assignment.my_scope().remove_node(assignment)
+                    self.optimizations_performed = True
                     self.num_warnings += 1
                     print_warning("{}: removed statement that has no effect".format(assignment.sourceref))
             if isinstance(assignment, AugAssignment):
@@ -169,22 +209,26 @@ class Optimizer:
                             self.num_warnings += 1
                             print_warning("{}: removed statement that has no effect".format(assignment.sourceref))
                             assignment.my_scope().remove_node(assignment)
+                            self.optimizations_performed = True
                         elif assignment.operator == "*=":
                             self.num_warnings += 1
                             print_warning("{}: statement replaced by = 0".format(assignment.sourceref))
                             new_assignment = self._make_new_assignment(assignment, 0)
                             assignment.my_scope().replace_node(assignment, new_assignment)
+                            self.optimizations_performed = True
                         elif assignment.operator == "**=":
                             self.num_warnings += 1
                             print_warning("{}: statement replaced by = 1".format(assignment.sourceref))
                             new_assignment = self._make_new_assignment(assignment, 1)
                             assignment.my_scope().replace_node(assignment, new_assignment)
+                            self.optimizations_performed = True
                     if assignment.right.value >= 8 and assignment.operator in ("<<=", ">>="):
                         print("{}: shifting result is always zero".format(assignment.sourceref))
                         new_stmt = Assignment(sourceref=assignment.sourceref)
                         new_stmt.nodes.append(AssignmentTargets(nodes=[assignment.left], sourceref=assignment.sourceref))
                         new_stmt.nodes.append(LiteralValue(value=0, sourceref=assignment.sourceref))
                         assignment.my_scope().replace_node(assignment, new_stmt)
+                        self.optimizations_performed = True
                     if assignment.operator in ("+=", "-=") and 0 < assignment.right.value < 256:
                         howmuch = assignment.right
                         if howmuch.value not in (0, 1):
@@ -194,10 +238,12 @@ class Optimizer:
                                             howmuch=howmuch.value, sourceref=assignment.sourceref)
                         new_stmt.target = assignment.left
                         assignment.my_scope().replace_node(assignment, new_stmt)
+                        self.optimizations_performed = True
                     if assignment.right.value == 1 and assignment.operator in ("/=", "//=", "*="):
                         self.num_warnings += 1
                         print_warning("{}: removed statement that has no effect".format(assignment.sourceref))
                         assignment.my_scope().remove_node(assignment)
+                        self.optimizations_performed = True
 
     @no_type_check
     def _make_new_assignment(self, old_aug_assignment: AugAssignment, constantvalue: int) -> Assignment:
@@ -222,7 +268,7 @@ class Optimizer:
 
     @no_type_check
     def _make_incrdecr(self, old_stmt: AstNode, target: Union[TargetRegisters, Register, SymbolName, Dereference],
-                       howmuch: Union[int, float], operator: str) -> AugAssignment:
+                       howmuch: Union[int, float], operator: str) -> IncrDecr:
         a = IncrDecr(operator=operator, howmuch=howmuch, sourceref=old_stmt.sourceref)
         a.nodes.append(target)
         a.parent = old_stmt.parent
@@ -245,6 +291,7 @@ class Optimizer:
                                 print("{}: joined with previous assignment".format(assignment.sourceref))
                                 assignments[0].left.nodes.extend(assignment.left.nodes)
                                 scope.remove_node(assignment)
+                                self.optimizations_performed = True
                             rvalue = None
                             assignments.clear()
                     else:
@@ -263,8 +310,9 @@ class Optimizer:
                 lvalues = set(assignment.left.nodes)
                 if len(lvalues) != len(assignment.left.nodes):
                     print("{}: removed duplicate assignment targets".format(assignment.sourceref))
-                # @todo change order: first registers, then zp addresses, then non-zp addresses, then the rest (if any)
-                assignment.left.nodes = list(lvalues)
+                    # @todo change order: first registers, then zp addresses, then non-zp addresses, then the rest (if any)
+                    assignment.left.nodes = list(lvalues)
+                    self.optimizations_performed = True
 
     @no_type_check
     def remove_unused_subroutines(self) -> None:
@@ -337,6 +385,155 @@ class Optimizer:
                     node.my_scope().nodes.remove(node)
 
 
+def process_expression(expr: Expression, sourceref: SourceRef) -> Any:
+    # process/simplify all expressions (constant folding etc)
+    if expr.must_be_constant:
+        return process_constant_expression(expr, sourceref)
+    else:
+        return process_dynamic_expression(expr, sourceref)
+
+
+def process_constant_expression(expr: Any, sourceref: SourceRef) -> LiteralValue:
+    # the expression must result in a single (constant) value (int, float, whatever) wrapped as LiteralValue.
+    if isinstance(expr, (int, float, str, bool)):
+        raise TypeError("expr node should not be a python primitive value", expr, sourceref)
+    elif expr is None or isinstance(expr, LiteralValue):
+        return expr
+    elif isinstance(expr, SymbolName):
+        value = check_symbol_definition(expr.name, expr.my_scope(), expr.sourceref)
+        if isinstance(value, VarDef):
+            if value.vartype == VarType.MEMORY:
+                raise ExpressionEvaluationError("can't take a memory value, must be a constant", expr.sourceref)
+            value = value.value
+        if isinstance(value, Expression):
+            raise ExpressionEvaluationError("circular reference?", expr.sourceref)
+        elif isinstance(value, LiteralValue):
+            return value
+        elif isinstance(value, (int, float, str, bool)):
+            raise TypeError("symbol value node should not be a python primitive value", expr)
+        else:
+            raise ExpressionEvaluationError("constant symbol required, not {}".format(value.__class__.__name__), expr.sourceref)
+    elif isinstance(expr, AddressOf):
+        assert isinstance(expr.name, SymbolName)
+        value = check_symbol_definition(expr.name.name, expr.my_scope(), expr.sourceref)
+        if isinstance(value, VarDef):
+            if value.vartype == VarType.MEMORY:
+                if isinstance(value.value, LiteralValue):
+                    return value.value
+                else:
+                    raise ExpressionEvaluationError("constant literal value required", value.sourceref)
+            if value.vartype == VarType.CONST:
+                raise ExpressionEvaluationError("can't take the address of a constant", expr.name.sourceref)
+            raise ExpressionEvaluationError("address-of this {} isn't a compile-time constant"
+                                            .format(value.__class__.__name__), expr.name.sourceref)
+        else:
+            raise ExpressionEvaluationError("constant address required, not {}"
+                                            .format(value.__class__.__name__), expr.name.sourceref)
+    elif isinstance(expr, SubCall):
+        if isinstance(expr.target, SymbolName):      # 'function(1,2,3)'
+            funcname = expr.target.name
+            if funcname in math_functions or funcname in builtin_functions:
+                func_args = []
+                for a in (process_constant_expression(callarg.value, sourceref) for callarg in expr.arguments.nodes):
+                    if isinstance(a, LiteralValue):
+                        func_args.append(a.value)
+                    else:
+                        func_args.append(a)
+                func = math_functions.get(funcname, builtin_functions.get(funcname))
+                try:
+                    return LiteralValue(value=func(*func_args), sourceref=expr.arguments.sourceref)  # type: ignore
+                except Exception as x:
+                    raise ExpressionEvaluationError(str(x), expr.sourceref)
+            else:
+                raise ExpressionEvaluationError("can only use math- or builtin function", expr.sourceref)
+        elif isinstance(expr.target, Dereference):       # '[...](1,2,3)'
+            raise ExpressionEvaluationError("dereferenced value call is not a constant value", expr.sourceref)
+        elif type(expr.target) is int:    # '64738()'
+            raise ExpressionEvaluationError("immediate address call is not a constant value", expr.sourceref)
+        else:
+            raise NotImplementedError("weird call target", expr.target)
+    elif not isinstance(expr, Expression):
+        raise ExpressionEvaluationError("constant value required, not {}".format(expr.__class__.__name__), expr.sourceref)
+    if expr.unary:
+        left_sourceref = expr.left.sourceref if isinstance(expr.left, AstNode) else sourceref
+        expr.left = process_constant_expression(expr.left, left_sourceref)
+        if isinstance(expr.left, LiteralValue) and type(expr.left.value) in (int, float):
+            try:
+                if expr.operator == '-':
+                    return LiteralValue(value=-expr.left.value, sourceref=expr.left.sourceref)  # type: ignore
+                elif expr.operator == '~':
+                    return LiteralValue(value=~expr.left.value, sourceref=expr.left.sourceref)  # type: ignore
+                elif expr.operator in ("++", "--"):
+                    raise ValueError("incr/decr should not be an expression")
+                raise ValueError("invalid unary operator", expr.operator)
+            except TypeError as x:
+                raise ParseError(str(x), expr.sourceref) from None
+        raise ValueError("invalid operand type for unary operator", expr.left, expr.operator)
+    else:
+        left_sourceref = expr.left.sourceref if isinstance(expr.left, AstNode) else sourceref
+        expr.left = process_constant_expression(expr.left, left_sourceref)
+        right_sourceref = expr.right.sourceref if isinstance(expr.right, AstNode) else sourceref
+        expr.right = process_constant_expression(expr.right, right_sourceref)
+        if isinstance(expr.left, LiteralValue):
+            if isinstance(expr.right, LiteralValue):
+                return expr.evaluate_primitive_constants(expr.right.sourceref)
+            else:
+                raise ExpressionEvaluationError("constant literal value required on right, not {}"
+                                                .format(expr.right.__class__.__name__), right_sourceref)
+        else:
+            raise ExpressionEvaluationError("constant literal value required on left, not {}"
+                                            .format(expr.left.__class__.__name__), left_sourceref)
+
+
+def process_dynamic_expression(expr: Any, sourceref: SourceRef) -> Any:
+    # constant-fold a dynamic expression
+    if isinstance(expr, (int, float, str, bool)):
+        raise TypeError("expr node should not be a python primitive value", expr, sourceref)
+    elif expr is None or isinstance(expr, LiteralValue):
+        return expr
+    elif isinstance(expr, SymbolName):
+        try:
+            return process_constant_expression(expr, sourceref)
+        except ExpressionEvaluationError:
+            return expr
+    elif isinstance(expr, AddressOf):
+        try:
+            return process_constant_expression(expr, sourceref)
+        except ExpressionEvaluationError:
+            return expr
+    elif isinstance(expr, SubCall):
+        try:
+            return process_constant_expression(expr, sourceref)
+        except ExpressionEvaluationError:
+            if isinstance(expr.target, SymbolName):
+                check_symbol_definition(expr.target.name, expr.my_scope(), expr.target.sourceref)
+            return expr
+    elif isinstance(expr, Register):
+        return expr
+    elif isinstance(expr, Dereference):
+        if isinstance(expr.operand, SymbolName):
+            check_symbol_definition(expr.operand.name, expr.my_scope(), expr.operand.sourceref)
+        return expr
+    elif not isinstance(expr, Expression):
+        raise ParseError("expression required, not {}".format(expr.__class__.__name__), expr.sourceref)
+    if expr.unary:
+        left_sourceref = expr.left.sourceref if isinstance(expr.left, AstNode) else sourceref
+        expr.left = process_dynamic_expression(expr.left, left_sourceref)
+        try:
+            return process_constant_expression(expr, sourceref)
+        except ExpressionEvaluationError:
+            return expr
+    else:
+        left_sourceref = expr.left.sourceref if isinstance(expr.left, AstNode) else sourceref
+        expr.left = process_dynamic_expression(expr.left, left_sourceref)
+        right_sourceref = expr.right.sourceref if isinstance(expr.right, AstNode) else sourceref
+        expr.right = process_dynamic_expression(expr.right, right_sourceref)
+        try:
+            return process_constant_expression(expr, sourceref)
+        except ExpressionEvaluationError:
+            return expr
+
+
 def optimize(mod: Module) -> None:
     opt = Optimizer(mod)
     opt.optimize()
diff --git a/il65/plyparse.py b/il65/plyparse.py
index 469b55903..e5419a07b 100644
--- a/il65/plyparse.py
+++ b/il65/plyparse.py
@@ -10,7 +10,7 @@ import builtins
 import inspect
 import enum
 from collections import defaultdict
-from typing import Union, Generator, Tuple, Sequence, List, Optional, Dict, Any, no_type_check
+from typing import Union, Generator, Tuple, List, Optional, Dict, Any, no_type_check
 import attr
 from ply.yacc import yacc
 from .plylex import SourceRef, tokens, lexer, find_tok_column, print_warning
@@ -809,155 +809,6 @@ def coerce_constant_value(datatype: DataType, value: AstNode,
     return False, value
 
 
-def process_expression(expr: Expression, sourceref: SourceRef) -> Any:
-    # process/simplify all expressions (constant folding etc)
-    if expr.must_be_constant:
-        return process_constant_expression(expr, sourceref)
-    else:
-        return process_dynamic_expression(expr, sourceref)
-
-
-def process_constant_expression(expr: Any, sourceref: SourceRef) -> LiteralValue:
-    # the expression must result in a single (constant) value (int, float, whatever) wrapped as LiteralValue.
-    if isinstance(expr, (int, float, str, bool)):
-        raise TypeError("expr node should not be a python primitive value", expr, sourceref)
-    elif expr is None or isinstance(expr, LiteralValue):
-        return expr
-    elif isinstance(expr, SymbolName):
-        value = check_symbol_definition(expr.name, expr.my_scope(), expr.sourceref)
-        if isinstance(value, VarDef):
-            if value.vartype == VarType.MEMORY:
-                raise ExpressionEvaluationError("can't take a memory value, must be a constant", expr.sourceref)
-            value = value.value
-        if isinstance(value, Expression):
-            raise ExpressionEvaluationError("circular reference?", expr.sourceref)
-        elif isinstance(value, LiteralValue):
-            return value
-        elif isinstance(value, (int, float, str, bool)):
-            raise TypeError("symbol value node should not be a python primitive value", expr)
-        else:
-            raise ExpressionEvaluationError("constant symbol required, not {}".format(value.__class__.__name__), expr.sourceref)
-    elif isinstance(expr, AddressOf):
-        assert isinstance(expr.name, SymbolName)
-        value = check_symbol_definition(expr.name.name, expr.my_scope(), expr.sourceref)
-        if isinstance(value, VarDef):
-            if value.vartype == VarType.MEMORY:
-                if isinstance(value.value, LiteralValue):
-                    return value.value
-                else:
-                    raise ExpressionEvaluationError("constant literal value required", value.sourceref)
-            if value.vartype == VarType.CONST:
-                raise ExpressionEvaluationError("can't take the address of a constant", expr.name.sourceref)
-            raise ExpressionEvaluationError("address-of this {} isn't a compile-time constant"
-                                            .format(value.__class__.__name__), expr.name.sourceref)
-        else:
-            raise ExpressionEvaluationError("constant address required, not {}"
-                                            .format(value.__class__.__name__), expr.name.sourceref)
-    elif isinstance(expr, SubCall):
-        if isinstance(expr.target, SymbolName):      # 'function(1,2,3)'
-            funcname = expr.target.name
-            if funcname in math_functions or funcname in builtin_functions:
-                func_args = []
-                for a in (process_constant_expression(callarg.value, sourceref) for callarg in expr.arguments.nodes):
-                    if isinstance(a, LiteralValue):
-                        func_args.append(a.value)
-                    else:
-                        func_args.append(a)
-                func = math_functions.get(funcname, builtin_functions.get(funcname))
-                try:
-                    return LiteralValue(value=func(*func_args), sourceref=expr.arguments.sourceref)  # type: ignore
-                except Exception as x:
-                    raise ExpressionEvaluationError(str(x), expr.sourceref)
-            else:
-                raise ExpressionEvaluationError("can only use math- or builtin function", expr.sourceref)
-        elif isinstance(expr.target, Dereference):       # '[...](1,2,3)'
-            raise ExpressionEvaluationError("dereferenced value call is not a constant value", expr.sourceref)
-        elif type(expr.target) is int:    # '64738()'
-            raise ExpressionEvaluationError("immediate address call is not a constant value", expr.sourceref)
-        else:
-            raise NotImplementedError("weird call target", expr.target)
-    elif not isinstance(expr, Expression):
-        raise ExpressionEvaluationError("constant value required, not {}".format(expr.__class__.__name__), expr.sourceref)
-    if expr.unary:
-        left_sourceref = expr.left.sourceref if isinstance(expr.left, AstNode) else sourceref
-        expr.left = process_constant_expression(expr.left, left_sourceref)
-        if isinstance(expr.left, LiteralValue) and type(expr.left.value) in (int, float):
-            try:
-                if expr.operator == '-':
-                    return LiteralValue(value=-expr.left.value, sourceref=expr.left.sourceref)  # type: ignore
-                elif expr.operator == '~':
-                    return LiteralValue(value=~expr.left.value, sourceref=expr.left.sourceref)  # type: ignore
-                elif expr.operator in ("++", "--"):
-                    raise ValueError("incr/decr should not be an expression")
-                raise ValueError("invalid unary operator", expr.operator)
-            except TypeError as x:
-                raise ParseError(str(x), expr.sourceref) from None
-        raise ValueError("invalid operand type for unary operator", expr.left, expr.operator)
-    else:
-        left_sourceref = expr.left.sourceref if isinstance(expr.left, AstNode) else sourceref
-        expr.left = process_constant_expression(expr.left, left_sourceref)
-        right_sourceref = expr.right.sourceref if isinstance(expr.right, AstNode) else sourceref
-        expr.right = process_constant_expression(expr.right, right_sourceref)
-        if isinstance(expr.left, LiteralValue):
-            if isinstance(expr.right, LiteralValue):
-                return expr.evaluate_primitive_constants(expr.right.sourceref)
-            else:
-                raise ExpressionEvaluationError("constant literal value required on right, not {}"
-                                                .format(expr.right.__class__.__name__), right_sourceref)
-        else:
-            raise ExpressionEvaluationError("constant literal value required on left, not {}"
-                                            .format(expr.left.__class__.__name__), left_sourceref)
-
-
-def process_dynamic_expression(expr: Any, sourceref: SourceRef) -> Any:
-    # constant-fold a dynamic expression
-    if isinstance(expr, (int, float, str, bool)):
-        raise TypeError("expr node should not be a python primitive value", expr, sourceref)
-    elif expr is None or isinstance(expr, LiteralValue):
-        return expr
-    elif isinstance(expr, SymbolName):
-        try:
-            return process_constant_expression(expr, sourceref)
-        except ExpressionEvaluationError:
-            return expr
-    elif isinstance(expr, AddressOf):
-        try:
-            return process_constant_expression(expr, sourceref)
-        except ExpressionEvaluationError:
-            return expr
-    elif isinstance(expr, SubCall):
-        try:
-            return process_constant_expression(expr, sourceref)
-        except ExpressionEvaluationError:
-            if isinstance(expr.target, SymbolName):
-                check_symbol_definition(expr.target.name, expr.my_scope(), expr.target.sourceref)
-            return expr
-    elif isinstance(expr, Register):
-        return expr
-    elif isinstance(expr, Dereference):
-        if isinstance(expr.operand, SymbolName):
-            check_symbol_definition(expr.operand.name, expr.my_scope(), expr.operand.sourceref)
-        return expr
-    elif not isinstance(expr, Expression):
-        raise ParseError("expression required, not {}".format(expr.__class__.__name__), expr.sourceref)
-    if expr.unary:
-        left_sourceref = expr.left.sourceref if isinstance(expr.left, AstNode) else sourceref
-        expr.left = process_dynamic_expression(expr.left, left_sourceref)
-        try:
-            return process_constant_expression(expr, sourceref)
-        except ExpressionEvaluationError:
-            return expr
-    else:
-        left_sourceref = expr.left.sourceref if isinstance(expr.left, AstNode) else sourceref
-        expr.left = process_dynamic_expression(expr.left, left_sourceref)
-        right_sourceref = expr.right.sourceref if isinstance(expr.right, AstNode) else sourceref
-        expr.right = process_dynamic_expression(expr.right, right_sourceref)
-        try:
-            return process_constant_expression(expr, sourceref)
-        except ExpressionEvaluationError:
-            return expr
-
-
 def check_symbol_definition(name: str, scope: Scope, sref: SourceRef) -> Any:
     try:
         return scope.lookup(name)
diff --git a/reference.md b/reference.md
index 994b70e6a..43d2dbed6 100644
--- a/reference.md
+++ b/reference.md
@@ -25,7 +25,7 @@ which aims to provide many conveniences over raw assembly code (even when using
 - breakpoints, that let the Vice emulator drop into the monitor if execution hits them
 - source code labels automatically loaded in Vice emulator so it can show them in disassembly
 - conditional gotos
-- some code optimizations (such as not repeatedly loading the same value in a register) 
+- various code optimizations (code structure, logical and numerical expressions, ...) 
 - @todo: loops
 - @todo: memory block operations