<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def bfd(self, **kwargs):
 """Configure BFD for Interface. Args: name (str):
 name of the interface to configure (230/0/1 etc) int_type (str):
 interface type (gigabitethernet etc) tx (str):
 BFD transmit interval in milliseconds (300, 500, etc) rx (str):
 BFD receive interval in milliseconds (300, 500, etc) multiplier (str):
 BFD multiplier. (3, 7, 5, etc) delete (bool):
 True if BFD configuration should be deleted. Default value will be False if not specified. get (bool):
 Get config instead of editing config. (True, False) callback (function):
 A function executed upon completion of the method. The only parameter passed to `callback` will be the ``ElementTree`` `config`. Returns: Return value of `callback`. Raises: KeyError: if `tx`, `rx`, or `multiplier` is not passed. Examples: """

        int_type = str(kwargs.pop('int_type').lower())
        kwargs['name'] = str(kwargs.pop('name'))
        kwargs['min_tx'] = kwargs.pop('tx')
        kwargs['min_rx'] = kwargs.pop('rx')
        kwargs['delete'] = kwargs.pop('delete', False)
        callback = kwargs.pop('callback', self._callback)
        valid_int_types = ['gigabitethernet', 'tengigabitethernet',
                           'fortygigabitethernet', 'hundredgigabitethernet']

        if int_type not in valid_int_types:
            raise ValueError('int_type must be one of: %s' %
                             repr(valid_int_types))
        kwargs['int_type'] = int_type

        bfd_tx = self._bfd_tx(**kwargs)
        bfd_rx = self._bfd_rx(**kwargs)
        bfd_multiplier = self._bfd_multiplier(**kwargs)
        if kwargs.pop('get', False):
            return self._get_bfd(bfd_tx, bfd_rx, bfd_multiplier)
        config = pynos.utilities.merge_xml(bfd_tx, bfd_rx)
        config = pynos.utilities.merge_xml(config, bfd_multiplier)
        return callback(config)