Function: RETURN-FROM block-name value-form Evaluate the VALUE-FORM, returning its values from the lexically enclosing block BLOCK-NAME. This is constrained to be used only within the dynamic extent of the block.
Example:
(defunnot/1 (relation cont)
"Negation by failure: If you can't prove G, then (not G) true."
(with-undo-bindings
(call/1 relation
#'(lambda () (return-from not/1 nil)))
(funcall cont)))