Completeness theorem for parse-cr.
Theorem:
(defthm parse-cr-when-tree-match (implies (and (tree-match-element-p tree (element-rulename *cr*) *grammar*) (tree-terminatedp tree)) (equal (parse-cr (append (tree->string tree) rest-input)) (mv nil (tree-fix tree) (nat-list-fix rest-input)))))