isError and isNa are boolean

src/com/bytezone/diskbrowser/visicalc/Function.java

@@ -12,6 +12,7 @@ abstract class Function extends AbstractValue implements Iterable<Value>
 
   protected final Sheet parent;
   protected final Cell cell;
+
   protected String functionName;
   protected String functionText;
   protected String fullText;

src/com/bytezone/diskbrowser/visicalc/IsError.java

@@ -12,6 +12,12 @@ class IsError extends Function
     values.add (source);
   }
 
+  @Override
+  public boolean isBoolean ()
+  {
+    return true;
+  }
+
   @Override
   public void calculate ()
   {

src/com/bytezone/diskbrowser/visicalc/IsNa.java

@@ -12,6 +12,12 @@ public class IsNa extends Function
     values.add (source);
   }
 
+  @Override
+  public boolean isBoolean ()
+  {
+    return true;
+  }
+
   @Override
   public void calculate ()
   {