$20\mathrm{.}5\mathrm{.}4$: Write a method that returns a new map with merged key$-$value pairs.

$597714\mathrm{.}2988516.$qx$3$zqy$7$

Write a method that, given two maps, yields a map with all key$/$value associations that occur in either map. In the resulting map, a key is associated with the set $($of size $1$ or $2)$ containing the values to which the key is mapped in the given maps. Use a TreeMap so that the values will appear in order.