Utilities

%% Basic predicates (do not worry about the definitions at this stage):

%% Linear precedence:
lp([],Z,Z).
lp([A|X],Y,[A|Z]) :- lp(X,Y,Z).

%% Backtrack (go back and take the next choice):
backtrack :- fail.

%% Carry on after backtracking:
perform(X) :- call(X),!,fail.
perform(_).

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