How to write a procedure that takes integer to string map and produces string to set of integers map

It is supposed to take an integer and string as a map and return string and set of integers as a map. Say the input is (205 -> "ORD" 309 -> "ORD"); it returns ("ORD" -> {205,309}). Something like that.

map(int,string)myMap;

map(int,set)setMap;