Build a fast alist whose keys are the subtrees of X
(cons-subtrees x nil) builds a fast alist that associates each subtree of X with T, without duplication.
Function: cons-subtrees
(defun cons-subtrees (x al) (declare (xargs :guard t)) (cond ((atom x) al) ((hons-get x al) al) (t (cons-subtrees (car x) (cons-subtrees (cdr x) (hons-acons x t al))))))
