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foldlfamily of predicates is defined as follows, with V0 an initial value and V the final value of the folding operation:
foldl(G, [X_11, ..., X_1n], [X_21, ..., X_2n], ..., [X_m1, ..., X_mn], V0, V) :- call(G, X_11, ..., X_m1, V0, V1), call(G, X_12, ..., X_m2, V1, V2), ... call(G, X_1n, ..., X_mn, V<n-1>, V).
No implementation for a corresponding
foldr is given. A
implementation would consist in first calling reverse/2 on each of
the m input lists, then applying the appropriate
foldl. This is
actually more efficient than using a properly programmed-out
recursive algorithm that cannot be tail-call optimized.