code: func (r *Route) prepareVars(pairs ...string) (map[string]string, error) {
  	m, err := mapFromPairsToString(pairs...)
  	if err != nil {
  		return nil, err
  	}
  	return r.buildVars(m), nil
  }
docs: // prepareVars converts the route variable pairs into a map. If the route has a
  // BuildVarsFunc, it is invoked.