code: func iteratorMorphism(it graph.Iterator) morphism {
  	return morphism{
  		Reversal: func(ctx *pathContext) (morphism, *pathContext) { return iteratorMorphism(it), ctx },
  		Apply: func(in shape.Shape, ctx *pathContext) (shape.Shape, *pathContext) {
  			return join(iteratorShape{it}, in), ctx
  		},
  	}
  }
docs: // iteratorMorphism simply tacks the input iterator onto the chain.