code: func filterMorphism(filt []shape.ValueFilter) morphism {
  	return morphism{
  		Reversal: func(ctx *pathContext) (morphism, *pathContext) { return filterMorphism(filt), ctx },
  		Apply: func(in shape.Shape, ctx *pathContext) (shape.Shape, *pathContext) {
  			return shape.AddFilters(in, filt...), ctx
  		},
  	}
  }
docs: // filterMorphism is the set of nodes that passes filters.