diff --git a/src/com/bytezone/diskbrowser/infocom/CodeManager.java b/src/com/bytezone/diskbrowser/infocom/CodeManager.java
index 964f055..90dd716 100644
--- a/src/com/bytezone/diskbrowser/infocom/CodeManager.java
+++ b/src/com/bytezone/diskbrowser/infocom/CodeManager.java
@@ -15,9 +15,9 @@ import com.bytezone.diskbrowser.utilities.HexFormatter;
 
 class CodeManager extends AbstractFile
 {
-  Header header;
-  int codeSize;
-  Map<Integer, Routine> routines = new TreeMap<Integer, Routine> ();
+  private final Header header;
+  private int codeSize;
+  private final Map<Integer, Routine> routines = new TreeMap<Integer, Routine> ();
 
   public CodeManager (Header header)
   {
@@ -60,7 +60,7 @@ class CodeManager extends AbstractFile
     return blocks;
   }
 
-  public void addMissingRoutines ()
+  void addMissingRoutines ()
   {
     System.out.printf ("%nWalking the code block%n%n");
     int total = 0;
@@ -125,12 +125,12 @@ class CodeManager extends AbstractFile
     return text.toString ();
   }
 
-  public boolean containsRoutineAt (int address)
+  boolean containsRoutineAt (int address)
   {
     return (routines.containsKey (address));
   }
 
-  public void addCodeRoutines ()
+  void addCodeRoutines ()
   {
     List<Integer> routines = header.objectManager.getCodeRoutines ();
     System.out.println ("Adding " + routines.size () + " code routines");
@@ -138,7 +138,7 @@ class CodeManager extends AbstractFile
       addRoutine (address, 0);
   }
 
-  public void addActionRoutines ()
+  void addActionRoutines ()
   {
     List<Integer> routines = header.grammar.getActionRoutines ();
     System.out.println ("Adding " + routines.size () + " action routines");
@@ -146,7 +146,7 @@ class CodeManager extends AbstractFile
       addRoutine (address, 0);
   }
 
-  public Routine addRoutine (int address, int caller)
+  Routine addRoutine (int address, int caller)
   {
     if (address == 0) // stack-based call
       return null;
@@ -174,7 +174,7 @@ class CodeManager extends AbstractFile
     return r;
   }
 
-  public Routine getRoutine (int address)
+  Routine getRoutine (int address)
   {
     return routines.get (address);
   }
diff --git a/src/com/bytezone/diskbrowser/infocom/Header.java b/src/com/bytezone/diskbrowser/infocom/Header.java
index 3206aa1..4e6ea47 100755
--- a/src/com/bytezone/diskbrowser/infocom/Header.java
+++ b/src/com/bytezone/diskbrowser/infocom/Header.java
@@ -22,13 +22,13 @@ class Header extends InfocomAbstractFile
   int checksum;
   int stringPointer;
 
-  Abbreviations abbreviations;
-  Dictionary dictionary;
-  ObjectManager objectManager;
-  StringManager stringManager;
-  CodeManager codeManager;
-  Globals globals;
-  Grammar grammar;
+  final Abbreviations abbreviations;
+  final Dictionary dictionary;
+  final ObjectManager objectManager;
+  final StringManager stringManager;
+  final CodeManager codeManager;
+  final Globals globals;
+  final Grammar grammar;
 
   public Header (String name, byte[] buffer, Disk disk)
   {
diff --git a/src/com/bytezone/diskbrowser/visicalc/Cell.java b/src/com/bytezone/diskbrowser/visicalc/Cell.java
index bc32f61..f5f161a 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Cell.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Cell.java
@@ -195,7 +195,7 @@ class Cell implements Comparable<Cell>, Value
   @Override
   public boolean isError ()
   {
-    assert isValue () : "Cell type: " + type;
+    //    assert isValue () : "Cell type: " + type;
     return value.isError ();
   }
 
@@ -211,7 +211,12 @@ class Cell implements Comparable<Cell>, Value
   @Override
   public Value calculate ()
   {
-    assert type == CellType.VALUE;
+    if (!isValue ())
+    {
+      System.out.println ("Not a value");
+      return this;
+    }
+    assert isValue () : "Cell type: " + type + " @ " + address;
     if (expressionText == null)
     {
       System.out.printf ("%s null expression text %n", address);
diff --git a/src/com/bytezone/diskbrowser/visicalc/Expression.java b/src/com/bytezone/diskbrowser/visicalc/Expression.java
index c382154..bbad68d 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Expression.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Expression.java
@@ -124,7 +124,7 @@ class Expression implements Value
   @Override
   public Value calculate ()
   {
-    //    System.out.printf ("Calculating: %s%n", text);
+    System.out.printf ("Calculating: %s%n", text);
     //    if (text.equals ("@NA"))
     //      Utility.printStackTrace ();