diff --git a/src/com/bytezone/diskbrowser/visicalc/Abs.java b/src/com/bytezone/diskbrowser/visicalc/Abs.java
index d306528..2154f53 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Abs.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Abs.java
@@ -9,8 +9,8 @@ public class Abs extends ValueFunction
   }
 
   @Override
-  void setValue ()
+  double calculateValue ()
   {
-    value = Math.abs (source.getValue ());
+    return Math.abs (source.getValue ());
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/AbstractValue.java b/src/com/bytezone/diskbrowser/visicalc/AbstractValue.java
index cf0972c..6879280 100644
--- a/src/com/bytezone/diskbrowser/visicalc/AbstractValue.java
+++ b/src/com/bytezone/diskbrowser/visicalc/AbstractValue.java
@@ -1,9 +1,10 @@
 package com.bytezone.diskbrowser.visicalc;
 
 import java.util.ArrayList;
+import java.util.Iterator;
 import java.util.List;
 
-public abstract class AbstractValue implements Value
+public abstract class AbstractValue implements Value, Iterable<Value>
 {
   protected final String typeText;
   protected double value;
@@ -71,6 +72,12 @@ public abstract class AbstractValue implements Value
     return false;
   }
 
+  @Override
+  public Iterator<Value> iterator ()
+  {
+    return values.iterator ();
+  }
+
   // for debugging
   String getValueText (int depth)
   {
diff --git a/src/com/bytezone/diskbrowser/visicalc/Acos.java b/src/com/bytezone/diskbrowser/visicalc/Acos.java
index 7f5355a..20be749 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Acos.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Acos.java
@@ -5,13 +5,12 @@ public class Acos extends ValueFunction
   Acos (Cell cell, String text)
   {
     super (cell, text);
-
     assert text.startsWith ("@ACOS(") : text;
   }
 
   @Override
-  public void setValue ()
+  public double calculateValue ()
   {
-    value = Math.acos (source.getValue ());
+    return Math.acos (source.getValue ());
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Asin.java b/src/com/bytezone/diskbrowser/visicalc/Asin.java
index f2afd45..9cdb7d6 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Asin.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Asin.java
@@ -9,8 +9,8 @@ public class Asin extends ValueFunction
   }
 
   @Override
-  public void setValue ()
+  public double calculateValue ()
   {
-    value = Math.asin (source.getValue ());
+    return Math.asin (source.getValue ());
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Atan.java b/src/com/bytezone/diskbrowser/visicalc/Atan.java
index 32e00ea..0a25cd3 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Atan.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Atan.java
@@ -9,8 +9,8 @@ public class Atan extends ValueFunction
   }
 
   @Override
-  public void setValue ()
+  public double calculateValue ()
   {
-    value = Math.atan (source.getValue ());
+    return Math.atan (source.getValue ());
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Condition.java b/src/com/bytezone/diskbrowser/visicalc/Condition.java
index 35aa6f9..0283262 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Condition.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Condition.java
@@ -96,7 +96,7 @@ class Condition extends AbstractValue implements Iterable<Value>
   @Override
   public String toString ()
   {
-    return String.format ("[cond=%s, op=%s, value=%s]", conditionText, comparator,
+    return String.format ("[cond=%s, op:%s, value=%s]", conditionText, comparator,
         valueText);
   }
 
diff --git a/src/com/bytezone/diskbrowser/visicalc/ConstantFunction.java b/src/com/bytezone/diskbrowser/visicalc/ConstantFunction.java
new file mode 100644
index 0000000..683f850
--- /dev/null
+++ b/src/com/bytezone/diskbrowser/visicalc/ConstantFunction.java
@@ -0,0 +1,9 @@
+package com.bytezone.diskbrowser.visicalc;
+
+public abstract class ConstantFunction extends Function
+{
+  public ConstantFunction (Cell cell, String text)
+  {
+    super (cell, text);
+  }
+}
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Cos.java b/src/com/bytezone/diskbrowser/visicalc/Cos.java
index 96cf3e8..3a401c2 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Cos.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Cos.java
@@ -9,8 +9,8 @@ public class Cos extends ValueFunction
   }
 
   @Override
-  public void setValue ()
+  public double calculateValue ()
   {
-    value = Math.cos (source.getValue ());
+    return Math.cos (source.getValue ());
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Error.java b/src/com/bytezone/diskbrowser/visicalc/Error.java
index ed69e6f..db3d8f2 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Error.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Error.java
@@ -1,6 +1,6 @@
 package com.bytezone.diskbrowser.visicalc;
 
-class Error extends Function
+class Error extends ConstantFunction
 {
   public Error (Cell cell, String text)
   {
diff --git a/src/com/bytezone/diskbrowser/visicalc/Exp.java b/src/com/bytezone/diskbrowser/visicalc/Exp.java
index de4f884..9158055 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Exp.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Exp.java
@@ -9,8 +9,8 @@ public class Exp extends ValueFunction
   }
 
   @Override
-  public void setValue ()
+  public double calculateValue ()
   {
-    value = Math.exp (source.getValue ());
+    return Math.exp (source.getValue ());
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Expression.java b/src/com/bytezone/diskbrowser/visicalc/Expression.java
index f388336..78f2a1a 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Expression.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Expression.java
@@ -1,10 +1,9 @@
 package com.bytezone.diskbrowser.visicalc;
 
 import java.util.ArrayList;
-import java.util.Iterator;
 import java.util.List;
 
-class Expression extends AbstractValue implements Iterable<Value>
+class Expression extends AbstractValue //implements Iterable<Value>
 {
   // Expressions:
   //   number
@@ -386,11 +385,11 @@ class Expression extends AbstractValue implements Iterable<Value>
     return text.toString ();
   }
 
-  @Override
-  public Iterator<Value> iterator ()
-  {
-    return values.iterator ();
-  }
+  //  @Override
+  //  public Iterator<Value> iterator ()
+  //  {
+  //    return values.iterator ();
+  //  }
 
   @Override
   public String toString ()
diff --git a/src/com/bytezone/diskbrowser/visicalc/False.java b/src/com/bytezone/diskbrowser/visicalc/False.java
index 30a106a..f5a076a 100644
--- a/src/com/bytezone/diskbrowser/visicalc/False.java
+++ b/src/com/bytezone/diskbrowser/visicalc/False.java
@@ -1,6 +1,6 @@
 package com.bytezone.diskbrowser.visicalc;
 
-public class False extends Function
+public class False extends ConstantFunction
 {
   False (Cell cell, String text)
   {
diff --git a/src/com/bytezone/diskbrowser/visicalc/Function.java b/src/com/bytezone/diskbrowser/visicalc/Function.java
index 6d50916..3a75f85 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Function.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Function.java
@@ -1,8 +1,6 @@
 package com.bytezone.diskbrowser.visicalc;
 
-import java.util.Iterator;
-
-abstract class Function extends AbstractValue implements Iterable<Value>
+abstract class Function extends AbstractValue
 {
   static final String[] functionList =
       { "@ABS(", "@ACOS(", "@AND(", "@ASIN(", "@ATAN(", "@AVERAGE(", "@COUNT(",
@@ -11,13 +9,10 @@ abstract class Function extends AbstractValue implements Iterable<Value>
         "@PI", "@SIN(", "@SUM(", "@SQRT(", "@TAN(", "@TRUE" };
 
   protected final Cell cell;
+  protected final String fullText;
 
-  protected String functionName;
-  protected String functionText;
-  protected String fullText;
-
-  protected Value source;
-  protected Range range;
+  protected final String functionName;
+  protected final String functionText;
 
   Function (Cell cell, String text)
   {
@@ -35,17 +30,11 @@ abstract class Function extends AbstractValue implements Iterable<Value>
     }
     else
     {
-      functionName = "";
+      functionName = text;
       functionText = "";
     }
   }
 
-  @Override
-  public Iterator<Value> iterator ()
-  {
-    return values.iterator ();
-  }
-
   @Override
   public String toString ()
   {
diff --git a/src/com/bytezone/diskbrowser/visicalc/Int.java b/src/com/bytezone/diskbrowser/visicalc/Int.java
index cf9359e..a004517 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Int.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Int.java
@@ -9,8 +9,8 @@ public class Int extends ValueFunction
   }
 
   @Override
-  public void setValue ()
+  public double calculateValue ()
   {
-    value = (int) source.getValue ();
+    return (int) source.getValue ();
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/IsError.java b/src/com/bytezone/diskbrowser/visicalc/IsError.java
index 0791937..619e527 100644
--- a/src/com/bytezone/diskbrowser/visicalc/IsError.java
+++ b/src/com/bytezone/diskbrowser/visicalc/IsError.java
@@ -15,8 +15,16 @@ class IsError extends ValueFunction
   }
 
   @Override
-  public void setValue ()
+  public void calculate ()
   {
-    value = source.isValueType (ValueType.ERROR) ? 1 : 0;
+    source.calculate ();
+    value = calculateValue ();
+    valueType = ValueType.VALUE;      // do not use source.getValueType()
+  }
+
+  @Override
+  public double calculateValue ()
+  {
+    return source.isValueType (ValueType.ERROR) ? 1 : 0;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/IsNa.java b/src/com/bytezone/diskbrowser/visicalc/IsNa.java
index d509e11..9fea7a1 100644
--- a/src/com/bytezone/diskbrowser/visicalc/IsNa.java
+++ b/src/com/bytezone/diskbrowser/visicalc/IsNa.java
@@ -15,8 +15,16 @@ public class IsNa extends ValueFunction
   }
 
   @Override
-  public void setValue ()
+  public void calculate ()
   {
-    value = source.isValueType (ValueType.NA) ? 1 : 0;
+    source.calculate ();
+    value = calculateValue ();
+    valueType = ValueType.VALUE;      // do not use source.getValueType()
+  }
+
+  @Override
+  public double calculateValue ()
+  {
+    return source.isValueType (ValueType.NA) ? 1 : 0;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Ln.java b/src/com/bytezone/diskbrowser/visicalc/Ln.java
index bb3ec1b..fa9c33d 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Ln.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Ln.java
@@ -9,8 +9,8 @@ public class Ln extends ValueFunction
   }
 
   @Override
-  public void setValue ()
+  public double calculateValue ()
   {
-    value = Math.log (source.getValue ());
+    return Math.log (source.getValue ());
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Log10.java b/src/com/bytezone/diskbrowser/visicalc/Log10.java
index 9fad0fc..bf94bb0 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Log10.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Log10.java
@@ -9,8 +9,8 @@ public class Log10 extends ValueFunction
   }
 
   @Override
-  public void setValue ()
+  public double calculateValue ()
   {
-    value = Math.log10 (source.getValue ());
+    return Math.log10 (source.getValue ());
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Na.java b/src/com/bytezone/diskbrowser/visicalc/Na.java
index 0fff1a5..9ed40bd 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Na.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Na.java
@@ -1,6 +1,6 @@
 package com.bytezone.diskbrowser.visicalc;
 
-public class Na extends Function
+public class Na extends ConstantFunction
 {
   public Na (Cell cell, String text)
   {
diff --git a/src/com/bytezone/diskbrowser/visicalc/Or.java b/src/com/bytezone/diskbrowser/visicalc/Or.java
index c54866d..6b3d293 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Or.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Or.java
@@ -2,7 +2,7 @@ package com.bytezone.diskbrowser.visicalc;
 
 class Or extends Function
 {
-  ConditionList conditions;
+  private final ConditionList conditions;
 
   public Or (Cell cell, String text)
   {
diff --git a/src/com/bytezone/diskbrowser/visicalc/Pi.java b/src/com/bytezone/diskbrowser/visicalc/Pi.java
index d7dcb3d..92e5575 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Pi.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Pi.java
@@ -1,15 +1,14 @@
 package com.bytezone.diskbrowser.visicalc;
 
-class Pi extends Function
+class Pi extends ConstantFunction
 {
   Pi (Cell cell, String text)
   {
     super (cell, text);
 
-    value = Math.PI;
-
     assert text.equals ("@PI") : text;
 
+    value = Math.PI;
     valueType = ValueType.VALUE;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Sin.java b/src/com/bytezone/diskbrowser/visicalc/Sin.java
index d53bf11..40824a5 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Sin.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Sin.java
@@ -9,8 +9,8 @@ public class Sin extends ValueFunction
   }
 
   @Override
-  public void setValue ()
+  public double calculateValue ()
   {
-    value = Math.sin (source.getValue ());
+    return Math.sin (source.getValue ());
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Sqrt.java b/src/com/bytezone/diskbrowser/visicalc/Sqrt.java
index a2ec445..cdf67ea 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Sqrt.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Sqrt.java
@@ -9,8 +9,8 @@ public class Sqrt extends ValueFunction
   }
 
   @Override
-  public void setValue ()
+  public double calculateValue ()
   {
-    value = Math.sqrt (source.getValue ());
+    return Math.sqrt (source.getValue ());
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Tan.java b/src/com/bytezone/diskbrowser/visicalc/Tan.java
index 74c9691..00f7d72 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Tan.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Tan.java
@@ -9,8 +9,8 @@ public class Tan extends ValueFunction
   }
 
   @Override
-  public void setValue ()
+  public double calculateValue ()
   {
-    value = Math.tan (source.getValue ());
+    return Math.tan (source.getValue ());
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/True.java b/src/com/bytezone/diskbrowser/visicalc/True.java
index 5622c1f..63fc3ac 100644
--- a/src/com/bytezone/diskbrowser/visicalc/True.java
+++ b/src/com/bytezone/diskbrowser/visicalc/True.java
@@ -1,6 +1,6 @@
 package com.bytezone.diskbrowser.visicalc;
 
-public class True extends Function
+public class True extends ConstantFunction
 {
   True (Cell cell, String text)
   {
diff --git a/src/com/bytezone/diskbrowser/visicalc/ValueFunction.java b/src/com/bytezone/diskbrowser/visicalc/ValueFunction.java
index f9ed9eb..deadb76 100644
--- a/src/com/bytezone/diskbrowser/visicalc/ValueFunction.java
+++ b/src/com/bytezone/diskbrowser/visicalc/ValueFunction.java
@@ -2,6 +2,8 @@ package com.bytezone.diskbrowser.visicalc;
 
 public abstract class ValueFunction extends Function
 {
+  protected Value source;
+
   ValueFunction (Cell cell, String text)
   {
     super (cell, text);
@@ -21,10 +23,9 @@ public abstract class ValueFunction extends Function
       return;
     }
 
-    //    value = Math.abs (source.getValue ());
-    setValue ();
+    value = calculateValue ();
     valueType = Double.isNaN (value) ? ValueType.ERROR : ValueType.VALUE;
   }
 
-  abstract void setValue ();
+  abstract double calculateValue ();
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/ValueListFunction.java b/src/com/bytezone/diskbrowser/visicalc/ValueListFunction.java
index e54e8d1..b1cf4b3 100644
--- a/src/com/bytezone/diskbrowser/visicalc/ValueListFunction.java
+++ b/src/com/bytezone/diskbrowser/visicalc/ValueListFunction.java
@@ -15,5 +15,4 @@ public abstract class ValueListFunction extends Function
     for (Value v : list)
       values.add (v);
   }
-
 }
\ No newline at end of file