Reporting errors at macro-expansion time

["David L. Andre" <DLA@jasper.scrc.symbolics.com>]

    Date: Mon, 11 Jul 88 22:27 EDT
    From: barmar@Think.COM (Barry Margolin)

	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.

Sorry.  I was unaware that the body of a LET was an implicit tagbody.