Is it a function?

A relation is a list of pairs. It is a function on a domain when every element of the domain appears exactly once as a first entry. You will test that and read off images.

The idea. Count how often each element of the domain appears as a first entry. Every count must be $1$.

Your tasks.

1. In is_function, return True if every element of domain appears exactly once as a first entry.
2. In image, return the second entry of the pair whose first entry is x.
3. Fill functions with the names in relations that are functions on $\{1, 2, 3\}$.