fix error message format for d-to-c translator
fix operator '?' so values returned are checked for correctness

If a user defines a new class X and defines a new comparison
operation method function for X?X, it should return one of the
following symbols: <, >, ==, incomparable.  This is now enforced,
to prevent confusion, in that an error message signals an incorrect
return value.
fix the documentation of the comparison operator x?y
adjust .gitignore files