Disambiguation between dec-val and hex-val.
Theorem: fail-dec-val-when-match-hex-val
(defthm fail-dec-val-when-match-hex-val (implies (and (tree-match-element-p tree (element-rulename *hex-val*) *grammar*) (tree-terminatedp tree)) (mv-nth 0 (parse-dec-val (append (tree->string tree) rest-input)))))