Basic theorems about faig-fix.
Theorem:
(defthm faig-eval-faig-fix (equal (faig-eval (faig-fix x) env) (faig-eval x env)))
Theorem:
(defthm faig-restrict-faig-fix (equal (faig-restrict (faig-fix x) al) (faig-restrict x al)))
Theorem:
(defthm faig-partial-eval-faig-fix (equal (faig-partial-eval (faig-fix x) al) (faig-partial-eval x al)))