code: func (r *Route) BuildVarsFunc(f BuildVarsFunc) *Route {
  	if r.buildVarsFunc != nil {
  		// compose the old and new functions
  		old := r.buildVarsFunc
  		r.buildVarsFunc = func(m map[string]string) map[string]string {
  			return f(old(m))
  		}
  	} else {
  		r.buildVarsFunc = f
  	}
  	return r
  }
docs: // BuildVarsFunc adds a custom function to be used to modify build variables
  // before a route's URL is built.