A push-down automaton written in Prolog

push-down.pl

uprg(P):- uprg(P, [#]).        % Creates a new push-down automaton

%Input | head of the stack :- Stackoutput
uprg([p|Tail], Stack):- uprg(Tail, [p|Stack]).
uprg([p|Tail], [#|Stack]):- uprg(Tail, [p, #|Stack]).
uprg([p|Tail], [a|Stack]):- uprg(Tail, [p, a|Stack]).

uprg([a|Tail], [p|Stack]):- uprg(Tail, [a|Stack]).
uprg([a|Tail], [a|Stack]):- uprg(Tail, [a|Stack]).
uprg([b|Tail], [a|Stack]):- uprg(Tail, [b|Stack]).
uprg([e|Tail], [b|Stack]):- uprg(Tail, Stack).

uprg([], [#]). %No more input and our last stack element is the default stack character -> valid