'Appends the event to the end of the sequence.
Args:
event: The polyphonic event to append to the end.
Raises:
ValueError: If `event` is not a valid polyphonic event.'

def append(self, event):

    if (not isinstance(event, PolyphonicEvent)):
        raise ValueError(('Invalid    polyphonic    event:    %s' % event))
    self._events.append(event)
