class Sass::Script::Parser

def special_fun
  first = try_tok(:special_fun)
  return square_list unless first
  str = literal_node(first.value, first.source_range)
  return str unless try_tok(:string_interpolation)
  mid = without_stop_at {assert_expr :expr}
  assert_tok :end_interpolation
  last = assert_expr(:special_fun)
  node(
    Tree::Interpolation.new(str, mid, last, false, false),
    first.source_range.start_pos)
end