functionObjects::subtract: From the first field subtract the remaining fields in the list

The operation can be applied to any volume or surface fields generating a
    volume or surface scalar field.

    Example of function object specification:
    \verbatim
    Tdiff
    {
        type            subtract;
        libs            ("libfieldFunctionObjects.so");
        fields          (T Tmean);
        result          Tdiff;
        executeControl  writeTime;
        writeControl    writeTime;
    }
    \endverbatim

src/functionObjects/field/fieldExpression/fieldExpression.C

@@ -39,13 +39,6 @@ namespace functionObjects
 
 // * * * * * * * * * * * * * Protected Member Functions  * * * * * * * * * * //
 
-bool Foam::functionObjects::fieldExpression::calc()
-{
-    NotImplemented;
-    return false;
-}
-
-
 void Foam::functionObjects::fieldExpression::setResultName
 (
     const word& typeName,

src/functionObjects/field/fieldExpression/fieldExpression.H

@@ -71,7 +71,7 @@ protected:
 
     // Protected member functions
 
-        virtual bool calc();
+        virtual bool calc() = 0;
 
         void setResultName(const word& typeName, const word& defaultArg);