def join(list1, list2, separator = Sass::Script::String.new("auto"))
assert_type separator, :String
unless %w[auto space comma].include?(separator.value)
raise ArgumentError.new("Separator name must be space, comma, or auto")
end
sep1 = list1.separator if list1.is_a?(Sass::Script::List) && !list1.value.empty?
sep2 = list2.separator if list2.is_a?(Sass::Script::List) && !list2.value.empty?
Sass::Script::List.new(
list1.to_a + list2.to_a,
if separator.value == 'auto'
sep1 || sep2 || :space
else
separator.value.to_sym
end)
end