code: func TagValueIn(t TagKey, err error) (value interface{}, ok bool) {
  	Walk(err, func(err error) bool {
  		if sc, isSC := err.(stackContexter); isSC {
  			if value, ok = sc.stackContext().tags[t]; ok {
  				return false
  			}
  		}
  		return true
  	})
  	return
  }
docs: // TagValueIn will retrieve the tagged value from the error that's associated
  // with this key, and a boolean indicating if the tag was present or not.