:- check success powers(A,B,C)
   : ( list_nnegint(A), nnegint(B), var(C) )
   => ( list_nnegint(C), sorted(C) ).

:- check comp powers(A,B,C)
   : ( list_nnegint(A), nnegint(B), var(C) )
   + not_fails.

:- checked calls powers(A,B,C)
   : ( list_nnegint(A), nnegint(B), var(C) ).

