diff --git a/src/com/bytezone/diskbrowser/visicalc/Abs.java b/src/com/bytezone/diskbrowser/visicalc/Abs.java
index 9aa8a08..74dfff0 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Abs.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Abs.java
@@ -8,9 +8,10 @@ public class Abs extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     Expression exp = new Expression (parent, functionText);
     value = Math.abs (exp.getValue ());
+    return this;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/And.java b/src/com/bytezone/diskbrowser/visicalc/And.java
index e95b21a..d1c551a 100644
--- a/src/com/bytezone/diskbrowser/visicalc/And.java
+++ b/src/com/bytezone/diskbrowser/visicalc/And.java
@@ -9,8 +9,9 @@ class And extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     value = 0;
+    return this;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Cell.java b/src/com/bytezone/diskbrowser/visicalc/Cell.java
index 183f2d0..591d895 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Cell.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Cell.java
@@ -32,6 +32,7 @@ class Cell implements Comparable<Cell>, Value
     type = CellType.VALUE;            // default to VALUE, formatting may change it
   }
 
+  @Override
   public boolean isValue ()
   {
     return type == CellType.VALUE;
@@ -124,7 +125,7 @@ class Cell implements Comparable<Cell>, Value
         return justify (repeat, colWidth);
 
       case VALUE:
-        if (value.isError () || value.isNaN ())
+        if (value.isError () || value.isNotAvailable ())
           return justify (value.getText (), colWidth);
 
         Double thisValue = value.getValue ();
@@ -203,14 +204,14 @@ class Cell implements Comparable<Cell>, Value
   }
 
   @Override
-  public boolean isNaN ()
+  public boolean isNotAvailable ()
   {
     assert type == CellType.VALUE;
-    return value.isNaN ();
+    return value.isNotAvailable ();
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     assert type == CellType.VALUE;
     if (expressionText == null)
@@ -221,11 +222,12 @@ class Cell implements Comparable<Cell>, Value
     }
     else
     {
-      // should use Number or Cell for simple Values
+      // should use Number or Cell or Function for simple Values
       value = new Expression (parent, expressionText);
       value.calculate ();
       valueType = value.getValueType ();
     }
+    return this;
   }
 
   @Override
diff --git a/src/com/bytezone/diskbrowser/visicalc/Count.java b/src/com/bytezone/diskbrowser/visicalc/Count.java
index 0a98a6b..278bd02 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Count.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Count.java
@@ -11,13 +11,18 @@ class Count extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     value = 0;
+    valueType = ValueType.VALUE;
+
     for (Address address : range)
     {
       Cell cell = parent.getCell (address);
-      if (cell == null || cell.isError () || cell.isNaN ())
+      if (cell == null || cell.isNotAvailable ())
+        continue;
+
+      if (cell.isError ())
       {
         valueType = ValueType.ERROR;
         break;
@@ -26,5 +31,6 @@ class Count extends Function
       if (cell.getValue () != 0.0)
         value++;
     }
+    return this;
   }
 }
\ 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 de74c9d..f286f9b 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Error.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Error.java
@@ -14,9 +14,9 @@ class Error extends Function
   }
 
   @Override
-  public boolean isNaN ()
+  public boolean isNotAvailable ()
   {
-    return true;
+    return false;
   }
 
   @Override
@@ -26,7 +26,8 @@ class Error extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
+    return this;
   }
 }
\ 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 99e4650..e5af794 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Expression.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Expression.java
@@ -90,7 +90,9 @@ class Expression implements Value
           {
             String addressText = getAddressText (line.substring (ptr));
             ptr += addressText.length ();
-            values.add (parent.getCell (addressText));
+            Cell cell = parent.getCell (addressText);
+            if (cell != null)
+              values.add (parent.getCell (addressText));
           }
           else
           {
@@ -117,18 +119,19 @@ class Expression implements Value
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
+    //    System.out.println (this);
     try
     {
       Value thisValue = values.get (0);
       thisValue.calculate ();
       if (thisValue.isError ())
       {
-        valueType = ValueType.ERROR;
-        return;
+        valueType = thisValue.getValueType ();
+        return this;
       }
-      value = thisValue.getValue ();
+      value = thisValue.isNotAvailable () ? 0 : thisValue.getValue ();
 
       String sign = signs.get (0);
       if (sign.equals ("(-)"))
@@ -140,10 +143,11 @@ class Expression implements Value
         thisValue.calculate ();
         if (thisValue.isError ())
         {
-          valueType = ValueType.ERROR;
-          return;
+          valueType = thisValue.getValueType ();
+          return this;
         }
-        double nextValue = thisValue.getValue ();
+
+        double nextValue = thisValue.isNotAvailable () ? 0 : thisValue.getValue ();
 
         sign = signs.get (i);
         if (sign.equals ("(-)"))
@@ -167,6 +171,7 @@ class Expression implements Value
     {
       valueType = ValueType.ERROR;
     }
+    return this;
   }
 
   @Override
@@ -176,15 +181,15 @@ class Expression implements Value
   }
 
   @Override
-  public boolean isNaN ()
+  public boolean isValue ()
   {
-    return Double.isNaN (value);
+    return valueType == ValueType.VALUE;
   }
 
   @Override
-  public String getText ()
+  public boolean isNotAvailable ()
   {
-    return isNaN () ? "NaN" : isError () ? "Error" : "";
+    return valueType == ValueType.NA;
   }
 
   @Override
@@ -200,6 +205,12 @@ class Expression implements Value
     return value;
   }
 
+  @Override
+  public String getText ()
+  {
+    return isNotAvailable () ? "NA" : isError () ? "Error" : "";
+  }
+
   private String checkBrackets (String input)
   {
     String line = input.trim ();
@@ -220,7 +231,7 @@ class Expression implements Value
         System.out.printf ("**** Unbalanced brackets: left:%d, right:%d  ****%n",
                            leftBracket, rightBracket);
         System.out.println (input);
-        return "@ERROR()";
+        return "@ERROR";
       }
       //      System.out.printf ("Old expression:[%s]%n", line);
       while (rightBracket < leftBracket)
@@ -286,11 +297,18 @@ class Expression implements Value
     int ptr = 0;
     for (Value value : values)
     {
+      assert value != null;
       text.append (signs.get (ptr));
-      text.append (value.getValue ());
+      //      value.calculate ();
+      //      if (value.isValue ())
+      //        text.append (value.getValue ());
       if (ptr < operators.size ())
+      {
+        //        System.out.println (operators.get (ptr));
         text.append (operators.get (ptr++));
+      }
     }
+    //    System.out.println ("finished building");
 
     return text.toString ();
   }
diff --git a/src/com/bytezone/diskbrowser/visicalc/Function.java b/src/com/bytezone/diskbrowser/visicalc/Function.java
index 555390c..05f508d 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Function.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Function.java
@@ -31,6 +31,7 @@ abstract class Function implements Value
   private static final Pattern addressList = Pattern.compile ("\\(([^,]+(,[^,]+)*)\\)");
 
   protected final Sheet parent;
+  protected String functionName;
   protected String functionText;
 
   protected ValueType valueType;
@@ -97,9 +98,15 @@ abstract class Function implements Value
     // get function's parameter string
     int pos = text.indexOf ('(');
     if (pos >= 0)
+    {
+      functionName = text.substring (0, pos);
       functionText = text.substring (pos + 1, text.length () - 1);
+    }
     else
+    {
+      functionName = "";
       functionText = "";
+    }
   }
 
   @Override
@@ -108,6 +115,12 @@ abstract class Function implements Value
     return valueType;
   }
 
+  @Override
+  public boolean isValue ()
+  {
+    return valueType == ValueType.VALUE;
+  }
+
   @Override
   public boolean isError ()
   {
@@ -115,9 +128,9 @@ abstract class Function implements Value
   }
 
   @Override
-  public boolean isNaN ()
+  public boolean isNotAvailable ()
   {
-    return Double.isNaN (value);
+    return valueType == ValueType.NA;
   }
 
   @Override
@@ -130,7 +143,7 @@ abstract class Function implements Value
   @Override
   public String getText ()
   {
-    return isNaN () ? "NaN" : isError () ? "Error" : "";
+    return isNotAvailable () ? "" : isError () ? "Error" : "";
   }
 
   protected Range getRange (String text)
@@ -176,6 +189,6 @@ abstract class Function implements Value
   @Override
   public String toString ()
   {
-    return String.format ("Function: %s", functionText);
+    return String.format ("Function: %s %s", functionName, functionText);
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/If.java b/src/com/bytezone/diskbrowser/visicalc/If.java
index 0e7bfea..8cfa5f8 100644
--- a/src/com/bytezone/diskbrowser/visicalc/If.java
+++ b/src/com/bytezone/diskbrowser/visicalc/If.java
@@ -23,7 +23,7 @@ class If extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     valueType = ValueType.VALUE;
 
@@ -34,7 +34,7 @@ class If extends Function
         expTrue = new Expression (parent, textTrue);
         expTrue.calculate ();
       }
-      if (expTrue.isError () || expTrue.isNaN ())
+      if (expTrue.isError () || expTrue.isNotAvailable ())
         valueType = expTrue.getValueType ();
       else
         value = expTrue.getValue ();
@@ -46,11 +46,12 @@ class If extends Function
         expFalse = new Expression (parent, textFalse);
         expFalse.calculate ();
       }
-      if (expFalse.isError () || expFalse.isNaN ())
+      if (expFalse.isError () || expFalse.isNotAvailable ())
         valueType = expFalse.getValueType ();
       else
         value = expFalse.getValue ();
     }
+    return this;
   }
 
   @Override
diff --git a/src/com/bytezone/diskbrowser/visicalc/Int.java b/src/com/bytezone/diskbrowser/visicalc/Int.java
index 3e5dcf0..b3434a0 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Int.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Int.java
@@ -9,9 +9,10 @@ public class Int extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     Expression exp = new Expression (parent, functionText);
     value = (int) exp.getValue ();
+    return this;
   }
 }
\ 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 8913180..6ecfc8b 100644
--- a/src/com/bytezone/diskbrowser/visicalc/IsError.java
+++ b/src/com/bytezone/diskbrowser/visicalc/IsError.java
@@ -11,11 +11,12 @@ class IsError extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     expression.calculate ();
     //    value = expression.getValue ();
     valueType = expression.getValueType ();
     value = isError () ? 1 : 0;
+    return this;
   }
 }
\ 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 5c3fd7a..6ed200e 100644
--- a/src/com/bytezone/diskbrowser/visicalc/IsNa.java
+++ b/src/com/bytezone/diskbrowser/visicalc/IsNa.java
@@ -11,9 +11,10 @@ public class IsNa extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     value = expression.getValue ();
     valueType = expression.getValueType ();
+    return this;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Lookup.java b/src/com/bytezone/diskbrowser/visicalc/Lookup.java
index 708c28b..c120b78 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Lookup.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Lookup.java
@@ -20,13 +20,13 @@ class Lookup extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     source.calculate ();
-    if (source.isError () || source.isNaN ())
+    if (source.isError () || source.isNotAvailable ())
     {
       valueType = source.getValueType ();
-      return;
+      return this;
     }
     double sourceValue = source.getValue ();
 
@@ -44,5 +44,7 @@ class Lookup extends Function
         value = parent.getCell (target.nextColumn ()).getValue ();
       else
         value = parent.getCell (target.nextRow ()).getValue ();
+
+    return this;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Max.java b/src/com/bytezone/diskbrowser/visicalc/Max.java
index 96980a4..f8b990f 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Max.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Max.java
@@ -11,7 +11,7 @@ class Max extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     value = Double.MIN_VALUE;
     int totalChecked = 0;
@@ -19,13 +19,13 @@ class Max extends Function
     for (Address address : range)
     {
       Cell cell = parent.getCell (address);
-      if (cell == null)
+      if (cell == null || cell.isNotAvailable ())
         continue;
 
-      if (cell.isError () || cell.isNaN ())
+      if (cell.isError ())
       {
         valueType = ValueType.ERROR;
-        return;
+        break;
       }
 
       double temp = cell.getValue ();
@@ -35,8 +35,10 @@ class Max extends Function
     }
 
     if (totalChecked == 0)
-      valueType = ValueType.ERROR;
+      valueType = ValueType.NA;
     else
       valueType = ValueType.VALUE;
+
+    return this;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Min.java b/src/com/bytezone/diskbrowser/visicalc/Min.java
index 120a38f..8a7d82c 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Min.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Min.java
@@ -11,7 +11,7 @@ class Min extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     value = Double.MAX_VALUE;
     int totalChecked = 0;
@@ -19,13 +19,13 @@ class Min extends Function
     for (Address address : range)
     {
       Cell cell = parent.getCell (address);
-      if (cell == null)
+      if (cell == null || cell.isNotAvailable ())
         continue;
 
-      if (cell.isError () || cell.isNaN ())
+      if (cell.isError ())
       {
         valueType = ValueType.ERROR;
-        return;
+        break;
       }
 
       double temp = cell.getValue ();
@@ -35,8 +35,10 @@ class Min extends Function
     }
 
     if (totalChecked == 0)
-      valueType = ValueType.ERROR;
+      valueType = ValueType.NA;
     else
       valueType = ValueType.VALUE;
+
+    return this;
   }
 }
\ 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 055a9b0..12d40bb 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Na.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Na.java
@@ -14,7 +14,7 @@ public class Na extends Function
   }
 
   @Override
-  public boolean isNaN ()
+  public boolean isNotAvailable ()
   {
     return true;
   }
@@ -26,7 +26,8 @@ public class Na extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
+    return this;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Npv.java b/src/com/bytezone/diskbrowser/visicalc/Npv.java
index 2def280..f2be91c 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Npv.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Npv.java
@@ -21,7 +21,7 @@ public class Npv extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     value = 0;
     valueType = ValueType.VALUE;
@@ -29,16 +29,18 @@ public class Npv extends Function
     for (Address address : range)
     {
       Cell cell = parent.getCell (address);
-      if (cell == null)
+      if (cell == null || cell.isNotAvailable ())
         continue;
 
-      if (cell.isError () || cell.isNaN ())
+      if (cell.isError ())
       {
-        valueType = ValueType.VALUE;
-        return;
+        valueType = ValueType.ERROR;
+        break;
       }
 
       double temp = cell.getValue ();
     }
+
+    return this;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Number.java b/src/com/bytezone/diskbrowser/visicalc/Number.java
index 857c1b9..b0fcaac 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Number.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Number.java
@@ -10,6 +10,7 @@ class Number implements Value
     try
     {
       value = Double.parseDouble (text);
+      valueType = ValueType.VALUE;
     }
     catch (NumberFormatException e)
     {
@@ -17,6 +18,12 @@ class Number implements Value
     }
   }
 
+  @Override
+  public boolean isValue ()
+  {
+    return valueType == ValueType.VALUE;
+  }
+
   @Override
   public boolean isError ()
   {
@@ -24,9 +31,9 @@ class Number implements Value
   }
 
   @Override
-  public boolean isNaN ()
+  public boolean isNotAvailable ()
   {
-    return Double.isNaN (value);
+    return valueType == ValueType.NA;
   }
 
   @Override
@@ -44,17 +51,18 @@ class Number implements Value
   @Override
   public String getText ()
   {
-    return null;
+    return valueType == ValueType.ERROR ? "Error" : "";
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
+    return this;
   }
 
   @Override
   public ValueType getValueType ()
   {
-    return null;
+    return valueType;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Or.java b/src/com/bytezone/diskbrowser/visicalc/Or.java
index 61751c2..fc5b7cf 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Or.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Or.java
@@ -9,8 +9,9 @@ class Or extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     value = 0;
+    return this;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Pi.java b/src/com/bytezone/diskbrowser/visicalc/Pi.java
index 5739787..7eed6ed 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Pi.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Pi.java
@@ -8,8 +8,9 @@ class Pi extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     value = 3.1415926536;
+    return this;
   }
 }
diff --git a/src/com/bytezone/diskbrowser/visicalc/Sum.java b/src/com/bytezone/diskbrowser/visicalc/Sum.java
index 5d2c878..d615e92 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Sum.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Sum.java
@@ -11,7 +11,7 @@ class Sum extends Function
   }
 
   @Override
-  public void calculate ()
+  public Value calculate ()
   {
     value = 0;
     valueType = ValueType.VALUE;
@@ -19,15 +19,18 @@ class Sum extends Function
     for (Address address : range)
     {
       Cell cell = parent.getCell (address);
-      if (cell == null)
+      if (cell == null || cell.isNotAvailable ())
         continue;
 
-      if (cell.isError () || cell.isNaN ())
+      if (cell.isError ())
       {
         valueType = ValueType.ERROR;
-        return;
+        break;
       }
+
       value += cell.getValue ();
     }
+
+    return this;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Value.java b/src/com/bytezone/diskbrowser/visicalc/Value.java
index f8f05b8..fccac5e 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Value.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Value.java
@@ -13,9 +13,11 @@ interface Value
 
   public String getText ();
 
+  public boolean isValue ();
+
   public boolean isError ();
 
-  public boolean isNaN ();
+  public boolean isNotAvailable ();
 
-  public void calculate ();
+  public Value calculate ();
 }
\ No newline at end of file