doc about range step value has to be a constant

docs/source/syntaxreference.rst

@@ -450,7 +450,8 @@ from the starting value to (and including) the ending value::
     <start>  downto  <end>   [ step  <step> ]
 
 You an provide a step value if you need something else than the default increment which is one (or,
-in case of downto, a decrement of one).   Because a step of minus one is so common you can just use
+in case of downto, a decrement of one).  Unlike the start and end values, the step value must be a constant.
+Because a step of minus one is so common you can just use
 the downto variant to avoid having to specify the step as well::
 
     0 to 7                   ; range of values 0, 1, 2, 3, 4, 5, 6, 7
@@ -729,6 +730,7 @@ for loop
 The loop variable must be a byte or word variable, and it must be defined separately first.
 The expression that you loop over can be anything that supports iteration (such as ranges like ``0 to 100``,
 array variables and strings) *except* floating-point arrays (because a floating-point loop variable is not supported).
+Remember that a step value in a range must be a constant value.
 
 You can use a single statement, or a statement block like in the example below::
 

docs/source/todo.rst

@@ -58,6 +58,7 @@ Libraries:
 
 Optimizations:
 
+- treat every scalar variable decl with initialization value, as const by default, unless the variable gets assigned to somewhere (or has its address taken, or is @shared)
 - VariableAllocator: can we think of a smarter strategy for allocating variables into zeropage, rather than first-come-first-served?
   for instance, vars used inside loops first, then loopvars, then uwords used as pointers, then the rest
 - various optimizers skip stuff if compTarget.name==VMTarget.NAME.  Once 6502-codegen is done from IR code,

examples/cx16/chunkedfile/mcf.p8

@@ -179,7 +179,7 @@ processchunk_call           jsr  $ffff      ; modified
         cx16.r3 = address
         while size {
             ubyte readsize = 255
-            if size < 255
+            if msb(size)==0
                 readsize = lsb(size)
             cx16.r2 = cx16.macptr(readsize, bonkbuffer, false)  ; can't macptr directly to bonk ram
             cx16.rombank(bonk)