delete_all can succeedin any of three ways...

delete_all([], E, []).

delete_all([E|Tail], E, Res) :- delete_all(Tail, E, Res).

delete_all([Head|Tail], E, [Head|Res]) :- delete_all(Tail, E, Res).

• Order in file only tells which rules are attempted first—later matching rules can be used after backtracking!

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