Returns a new collection containing the result of applying fn to each
member of collection, which may be a set, bag, or seq. In the bag case,
the multiplicity of each member of the result is the sum of the multiplicities
of the values that fn maps to it. 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). collection can also be a map, in which case fn must be a
Lisp function of two arguments that returns two values (the map-default of the
result is that of collection).