class Array

def flatten_with_optional_argument!(level=-1)

Adapted from rubinius'
made, returns nil, otherwise self.
Flattens self in place as #flatten. If no changes are

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