(delta '(((a b) ()) (c (d (d))))) ; ~~> #t
(delta '(((a b c) ()) (c (d ())))) ; ~~> #t
(delta '(((a b) ()) (c (d ()) c))) ; ~~> #f
(delta '(((a b) ()) (c (d (d))))) ; ~~> #t
(delta '(((a b c) ()) (c (d ())))) ; ~~> #t
(delta '(((a b) ()) (c (d ()) c))) ; ~~> #f