4. Recall that a function f is onto if, and only if, for every element y
in the co-domain, there exists an element x in the domain such that f(x) = y.
A function f is not onto if, and only if, there exists an element y in the co-domain such
that for all elements x in the domain, f(x) is not equal to y.
The trick to this question is to look at the domain and co-domains carefully.