code: func (g *Graph) Copy(c *Graph) {
  	if c == nil {
  		return
  	}
  	for _, n := range c.Out {
  		nn := g.Add(n.This)
  		nn.Copy(n)
  	}
  }
docs: // Copy adds a copy of the graph given to the current graph.
  //
  // Warning (from the Go faq): Copying an interface value makes a copy of the
  // thing stored in the interface value. If the interface value holds a struct,
  // copying the interface value makes a copy of the struct. If the interface
  // value holds a pointer, copying the interface value makes a copy of the
  // pointer, but not the data it points to.