code: func (f *fieldVal) Negate(magnitude uint32) *fieldVal {
  	return f.NegateVal(f, magnitude)
  }
docs: // Negate negates the field value.  The existing field value is modified.  The
  // caller must provide the magnitude of the field value for a correct result.
  //
  // The field value is returned to support chaining.  This enables syntax like:
  // f.Negate().AddInt(1) so that f = -f + 1.