MapSet.union

You're seeing just the function union, go back to MapSet module for more information.

union(map_set1, map_set2)

union(t(val1), t(val2)) :: t(val1 | val2) when val1: value(), val2: value()

Returns a set containing all members of map_set1 and map_set2.

iex> MapSet.union(MapSet.new([1, 2]), MapSet.new([2, 3, 4]))
#MapSet<[1, 2, 3, 4]>