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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def hold(self, policy="combine"):
        ''' Activate a document hold.

        While a hold is active, no model changes will be applied, or trigger
        callbacks. Once ``unhold`` is called, the events collected during the
        hold will be applied according to the hold policy.

        Args:
            hold ('combine' or 'collect', optional)
                Whether events collected during a hold should attempt to be
                combined (default: 'combine')

                When set to ``'collect'`` all events will be collected and
                replayed in order as-is when ``unhold`` is called.

                When set to ``'combine'`` Bokeh will attempt to combine
                compatible events together. Typically, different events that
                change the same property on the same mode can be combined.
                For example, if the following sequence occurs:

                .. code-block:: python

                    doc.hold('combine')
                    slider.value = 10
                    slider.value = 11
                    slider.value = 12

                Then only *one* callback, for the last ``slider.value = 12``
                will be triggered.

        Returns:
            None

        .. note::
            ``hold`` only applies to document change events, i.e. setting
            properties on models. It does not apply to events such as
            ``ButtonClick``, etc.

        '''
        if self._hold is not None and self._hold != policy:
            log.warning("hold already active with '%s', ignoring '%s'" % (self._hold, policy))
            return
        if policy not in HoldPolicy:
            raise ValueError("Unknown hold policy %r" % policy)
        self._hold = policy