Takes two "count maps" (i.
Takes two "count maps" (i.e. Map values that contain numeric counts as the values of the keys), and does a point-wise add. For example,
add(Map("A"->1, "B"->1), Map("B"->1, "C"->1))
should be equal to
Map("A"->1, "B"->2, "C"->1)
The operation should be commutative, although it contains a "filter" step on the second addend (in the current implementation), so if the two maps are of wildly different sizes, you might want to make the larger map the first argument.
Left addend
Right addend