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Reporting errors at macro-expansion time

    Date: Mon, 11 Jul 88 13:34 EDT
    From: cjl@wheaties.ai.mit.edu (Chris Lindblad)

    What I do is use warn instead of error, and have the macro expand to something
    safe, like nil.

NIL isn't safe.  Expand into (PROGN) or (VALUES) instead.  If the form
is in the body of a TAGBODY (or anything with an implicit tag body, such
as DO or PROG), NIL is treated as a tag, not a form, and if you have
multiple of them you will get a warning about duplicate tags.