code: func (g *Graph) Substitute(s string, v interface{}) {
  	if g == nil || g.Out == nil {
  		return
  	}
  	for _, n := range g.Out {
  		if _string(n.This) == s {
  			n.This = v
  		}
  		n.Substitute(s, v)
  	}
  
  }
docs: // Substitute traverses the graph substituting all nodes with content
  // equal to s by v.