<programming> A feature of some implementations of unification which causes unification of a logic variable V and a structure S to fail if S contains V. Binding a variable to a structure containing that variable results in a cyclic structure which may subsequently cause unification to loop forever. Some implementations use extra pointer comparisons to avoid this. Most implementations of Prolog do not perform the occurs check for reasons of efficiency. Without occurs check the complexity of unification is O(min(size(term1), size(term2))) with occurs check it's O(max(size(term1), size(term2))) In theorem proving unification without the occurs check can lead to unsound inference. For example, in Prolog it is quite valid to write X = f(X). which will succeed, binding X to a cyclic structure. Clearly however, if f is taken to stand for a function rather than a constructor, then the above equality is only valid if f is the identity function. Weijland calls unification without occur check, "complete unification". The reference below describes a complete unification algorithm in terms of Colmerauer's consistency algorithm. ["Semantics for Logic Programs without Occur Check", W.P. Weijland, Theoretical Computer Science 71 (1990) pp 155-174]. (1996-01-11)

In computer science, the occurs check is a part of algorithms for syntactic unification. It causes unification of a variable V and a structure S to fail if S contains V.

n. a check issued by a bank on its own account for the amount paid to the bank by the purchaser with a named payee, and stating the name of the party purchasing the check (the remitter). The check is received as cash since it is guaranteed by the bank and does not depend on the account of a private individual or business. Cashiers' checks are commonly used when payment must be credited immediately upon receipt for business, real estate transfers, tax payments and the like.