% pretty print a prolog structure
pp(P):-pp([],P).
pp(L,P):-atomic(P),indented_print(L,P).
pp(L,P):-var(P),indented_print(L,P).
pp(L,[H|T]):-indented_print(L,'['), pp(['   |'|L],H),ppa(['   |'|L],T),indented_print(L,']').
pp(L,P):-P=..[F|Arg], pp(L,F),  ppa(['   |'|L], Arg).

ppa(L,[H|T]):-pp(L,H),ppa(L,T).
ppa(L,[]):-true.

indented_print([H|T], P):-write(H), indented_print(T, P).
indented_print([],P):-write('-'), write(P), nl.