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