code: func (p *Path) SaveReverse(via interface{}, tag string) *Path {
  	np := p.clone()
  	np.stack = append(np.stack, saveReverseMorphism(via, tag))
  	return np
  }
docs: // SaveReverse is the same as Save, only in the reverse direction
  // (the subject of the linkage should be tagged, instead of the object).