code: def pad(obj, pad_length):
      
      _check_supported(obj)
      copied = deepcopy(obj)
      copied.pad(pad_length)
      return copied
docs: Return a copy of the object with piano-roll padded with zeros at the end
      along the time axis.
  
      Parameters
      ----------
      pad_length : int
          The length to pad along the time axis with zeros.