'Create an update document and add it to the list of ops.'

def add_update(self, selector, update, multi=False, upsert=False, collation=None):

    validate_ok_for_update(update)
    cmd = SON([('q', selector), ('u', update), ('multi', multi), ('upsert', upsert)])
    collation = validate_collation_or_none(collation)
    if (collation is not None):
        self.uses_collation = True
        cmd['collation'] = collation
    self.ops.append((_UPDATE, cmd))
