once-only (specs &body forms)

Evaluates forms with symbols specified in specs rebound to temporary variables, ensuring that each initform is evaluated only once.

Each of specs must either be a symbol naming the variable to be rebound, or of the form:

 (symbol initform)  

Bare symbols in specs are equivalent to

 (symbol symbol)  

 (defmacro cons1 (x) (once-only (x) `(cons ,x ,x))) => CONS1


 (let ((y 0)) (cons1 (incf y))) => (1 . 1)