In Prolog, define the prefix predicate so that prefix(X,Y) says that X is a list that is a prefix of Y. That is, each element of X is equal to (unifies with) the corresponding element of Y, but Y may contain additional elements after tha. Check that your predicate works when X is uninstantiated: given a query like prefix(X, [1,2,3]), it should find all the prefixes of the list [1,2,3].