code: func findPlayCode(e present.Elem) []*present.Code {
  	var r []*present.Code
  	switch v := e.(type) {
  	case present.Code:
  		if v.Play {
  			r = append(r, &v)
  		}
  	case present.Section:
  		for _, s := range v.Elem {
  			r = append(r, findPlayCode(s)...)
  		}
  	}
  	return r
  }
docs: // findPlayCode returns a slide with all the Code elements in the given
  // Elem with Play set to true.