Map the values of a set: calls the map function f once for each
member of the set, returning an array which contains the result of
each one of those function calls. Note that since a set isn't
ordered, the values in the returned array will be in more or less
random order. If you need to know which member value produced which
result, you have to make that a part of what the filtering function
returns.
The filtering function f is called with a single, mixed-type
argument which is the member value to be mapped.