code: func (p *Path) In(via ...interface{}) *Path {
  	np := p.clone()
  	np.stack = append(np.stack, inMorphism(nil, via...))
  	return np
  }
docs: // In updates this Path to represent the nodes that are adjacent to the
  // current nodes, via the given inbound predicate.
  //
  // For example:
  //  // Return the list of nodes that follow "B".
  //  //
  //  // Will return []string{"A", "C", "D"} if there are the appropriate
  //  // edges from those nodes to "B" labelled "follows".
  //  StartPath(qs, "B").In("follows")