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LET-IF

    Date: Fri, 22 Nov 85 20:41:51 EST
    From: "George J. Carrette" <GJC@MIT-MC.ARPA>

    Funny you should mention that, I'm considering flushing quite a few
    special forms in the LMI system and replacing them with macros, LET-IF
    is one, and the following definition seems right:

    (defmacro let-if (pred bindings &body body)
      (let ((f (gentemp "f")))
       `(flet ((,f () ,@body))
	   (if ,pred (let ,bindings (,f)) (,f)))))

Unfortunately, this doesn't work if you want the bindings to be lexical
instead of special bindings.  Consider:

   (defun xor (a b)
     (let-if a ((b (not b)))
       b))

Unless B is special, this won't work using your scheme.