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not_less_than_zeroif called with a negative integer. E.g.
succ(X, 0)fails silently and
succ(X, -1)raises a domain error.111The behaviour to deal with natural numbers only was defined by Richard O'Keefe to support the common count-down-to-zero in a natural way. Up to 5.1.8, succ/2 also accepted negative integers.