:- op(300,xf,?).

begin:- asserta(definition(X,X)),
	asserta(definition((P;Q),[P, or, Q])),
	asserta(definition((P:-Q), [P, if, Q])),
	asserta(definition((P,Q), [P, and, Q])),
    prompt(Old, 'Prolog: '), repeat,nl,nl,read(T), (T=stop; process(T), fail).

process(T-D) :- !, asserta(definition(T,D)).
process([T1|T2]?) :- retrovert(T,[T1|T2]), !, nl, write(T).
process(T) :- translate(T,D), nl, output(D), !.
process(_) :- nl, write('*** untranslatable ***'), nl.

retrovert(T,D) :- definition(T1,D), !, T1 =.. [N|L],
		  retrovert_args(Args,L), T =.. [N|Args].

retrovert_args([A|Args],[L|Ls]) :- retrovert(A,L),
				   retrovert_args(Args, Ls).
retrovert_args([], []).

translate(X,X) :- var(X), !.
translate(T,D) :- T =.. [N|Args], args(Args,L),
		  T1 =.. [N|L], definition(T1,D).

args([A|Args], [L|Ls]) :- translate(A,L), args(Args, Ls).
args([], []).

output([X|Xs]) :- var(X), write(X), write(' '), output(S).
output([[X]|S]) :- output([X|S]).
output([[X|L]|S]) :- write('('), output([X|L]), write(')'), output(S).
output([X|S]) :- write(X), write(' '), output(S).
output([]).