For Mozart-Oz Programming. Can you help me understand this as well.

Write a function Position Xs Y] that returns the first position of Y in the list Xs. The positions in a list start with 1. For example, (Position [a b c] c) returns 3 and (Position [a b cb] b) returns 2. Try two versions: 1. 1) one that assumes that Y is an element of Xs and 2) one that returns 0, if Y does not occur in Xs