Making the most out of aggregate

    % count(Count, First, Last, Index):
    %
    % Count the number of values in an index, providing the first and last
    % indexes.
    %
:- pred count(int::out, int::out, int::out, T::in) is det.

count(C, F, L, T) :-
    ( A = func(I, U@{Count, First0, Last0}) = {Count + 1, First, Last} :-
        ( U = {0, 0, 0} ->
            First = I, Last = I
        ;
            First = (First0 < I -> First0 ; I),
            Last = (Last0 > I -> Last0 ; I)
        )
    ),
    (P = pred(I) :- index(I, T)),
    aggregate(P, A, {0, 0, 0}, {C, F, L}).