Returns a new collection containing those members or pairs of collection
for which fn returns true. If collection is a set, bag, or seq, fn is
called with one argument; if a map, fn is called with two arguments, the key
and the value (the map-default of the result is that of collection). As well
as a Lisp function, fn can be a map, or a set (which is treated as mapping
its members to true and everything else to false).