def union(self, other: 'FileLineSet') -> 'FileLineSet':
        """
        Returns a set of file lines that contains the union of the lines within
        this set and a given set.
        """
        # this isn't the most efficient implementation, but frankly, it doesn't
        # need to be.
        assert isinstance(other, FileLineSet)
        l_self = list(self)
        l_other = list(other)
        l_union = l_self + l_other
        return FileLineSet.from_list(l_union)
Returns a set of file lines that contains the union of the lines within
        this set and a given set.