code: def _op(self, operation, other, *allowed):
  		
  		
  		f = self._field
  		
  		if self._combining:  # We are a field-compound query fragment, e.g. (Foo.bar & Foo.baz).
  			return reduce(self._combining,
  					(q._op(operation, other, *allowed) for q in f))  # pylint:disable=protected-access
  		
  		# Optimize this away in production; diagnosic aide.
  		if __debug__ and _complex_safety_check(f, {operation} | set(allowed)):  # pragma: no cover
  			raise NotImplementedError("{self!r} does not allow {op} comparison.".format(self=self, op=operation))
  		
  		if other is not None:
  			other = f.transformer.foreign(other, (f, self._document))
  		
  		return Filter({self._name: {operation: other}})
docs: A basic operation operating on a single value.