Unification in append rule

append([],Ys,Ys).

append([X|Xs],Ys,[X|Zs]) :-

append(Xs,Ys,Zs).

append([1,2,3],A,[1,2,3,4]). results in[X|Xs] = [1,2,3], A=Ys, [X|Zs] = [1,2,3,4]

and thus:X=1, Xs=[2,3], Zs=[2,3,4]

so must prove: append([2,3],A,[2,3,4])

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