code: func (m *Mux) SetFuncMapToEngine(funcMap map[string]interface{}, e Engine) *Mux {
  	if len(funcMap) == 0 {
  		return m
  	}
  	// add the shared  funcs if template engine supports funcs.
  	if funcer, ok := e.(EngineFuncs); ok {
  		if funcer.Funcs() != nil {
  			for k, v := range funcMap {
  				funcer.Funcs()[k] = v
  			}
  		}
  	}
  	return m
  }
docs: // SetFuncMapToEngine sets or overrides a specific func map to a specific template engine
  // SharedFuncs stays untouched here.
  //
  // Call UseFuncMap if you want to set SharedFuncs and be applied
  // to all registered and future template engines registrations.