def hold(self, b=None):
if b is None: self._hold = not self._hold<EOL>else: self._hold = b<EOL>
Set the hold state.  If hold is None (default), toggle the
hold state.  Else set the hold state to boolean value b.

Eg::

    hold()      # toggle hold
    hold(True)  # hold is on
    hold(False) # hold is off