<div class="notebook">

<div class="nb-cell program" name="p1">
leftpad(Pad, To_Length, ListIn, ListOut) :-
    integer(To_Length),
    proper_length(ListIn, N),
    Dif is To_Length - N,
    a_prefix(Dif, Pad, Prefix),
    append(Prefix, ListIn, ListOut).

a_prefix(N,   _,         []) :- N =&lt; 0, !.
a_prefix(N, Pad, [Pad|Tail]) :- N &gt;  0,
    M is N - 1,
    a_prefix(M, Pad, Tail).

</div>

<div class="nb-cell query" name="q1">
leftpad(P, 5, [2, 3], Out).
</div>

<div class="nb-cell query" name="q2">
leftpad(P, 2, [2, 3, 7, 8], Out).
</div>

</div>