code: func labelsMorphism() morphism {
  	return morphism{
  		Reversal: func(ctx *pathContext) (morphism, *pathContext) {
  			panic("not implemented")
  		},
  		Apply: func(in shape.Shape, ctx *pathContext) (shape.Shape, *pathContext) {
  			return shape.Labels(in), ctx
  		},
  	}
  }
docs: // labelsMorphism iterates to the uniqified set of labels from
  // the given set of nodes in the path.