PPT Slide
Prove: ret (append (?, r)) = (ret ?) ? (ret r)
ret (append (< >, r)) = (ret <>) ? (ret r)
ret r = (ret <>) ? (ret r)
? <ret r = ran r and ran <> = ?>
Prove ret (append (<x> ^ ?, r)) = (ret (<x>)^?)) ? (ret r)
assuming ret (append (?, r)) = (ret ?) ? (ret r)