Macro Asif

Part of:

package anaphora
( asif < test > < then > &optional < else > )

Like IF, except binds the result of the test to IT (via LET) for
the the scope of the then-expression, and the test form to IT (via
SYMBOL-MACROLET) for the scope of the else-expression. Within scope of
the else-expression IT can be set with SETF.