# File lib/backports/1.8.7/array/flatten.rb, line 15
    def flatten_with_optional_argument!(level=-1)
      level = Backports.coerce_to_int(level)
      return flatten_without_optional_argument! if level < 0

      out = []
      ret = recursively_flatten_finite(self, out, level)
      replace(out) if ret
      ret
    end