Major Section: QUANTIFIERS
The following example illustrates the use of recursion as a means of
avoiding proof difficulties that can arise from the use of explicit
quantification (via defun-sk). See quantifiers for more about
the context of this example.
(in-package "ACL2"); We prove that if every member A of each of two lists satisfies the ; predicate (P A), then this holds of their append; and, conversely.
; Here is a solution using recursively-defined functions.
(defstub p (x) t)
(defun all-p (x) (if (atom x) t (and (p (car x)) (all-p (cdr x)))))
(defthm all-p-append (equal (all-p (append x1 x2)) (and (all-p x1) (all-p x2))))
