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Error Messages  The compiler writer has a serious obligation:
the compiler must produce either correct output code or an error
message.  Good error messages can save a great deal of
programmer time; this makes it worth the trouble to produce
them.    1. The message should be written out as text.    2. A
pointer to the point of the error in the input program should be
provided when appropriate.    3. Values from the program should
be included in the message where appropriate.    4. Diagnostic
messages ( e.g., unused variables) should be included, but user
should be able to turn them off.          X[CVAR] := 3.14
may not be used as a subscript.