Next unique placeholder name for placeholder shape of type *ph_type*,
        with id number *id* and orientation *orient*. Usually will be standard
        placeholder root name suffixed with id-1, e.g.
        _next_ph_name(ST_PlaceholderType.TBL, 4, 'horz') ==>
        'Table Placeholder 3'. The number is incremented as necessary to make
        the name unique within the collection. If *orient* is ``'vert'``, the
        placeholder name is prefixed with ``'Vertical '``.

def _next_ph_name(self, ph_type, id, orient):
        """
        Next unique placeholder name for placeholder shape of type *ph_type*,
        with id number *id* and orientation *orient*. Usually will be standard
        placeholder root name suffixed with id-1, e.g.
        _next_ph_name(ST_PlaceholderType.TBL, 4, 'horz') ==>
        'Table Placeholder 3'. The number is incremented as necessary to make
        the name unique within the collection. If *orient* is ``'vert'``, the
        placeholder name is prefixed with ``'Vertical '``.
        """
        basename = self.ph_basename(ph_type)

        # prefix rootname with 'Vertical ' if orient is 'vert'
        if orient == ST_Direction.VERT:
            basename = 'Vertical %s' % basename

        # increment numpart as necessary to make name unique
        numpart = id - 1
        names = self._spTree.xpath('//p:cNvPr/@name')
        while True:
            name = '%s %d' % (basename, numpart)
            if name not in names:
                break
            numpart += 1

        return name