code: func (o *Once) Do(f func()) {
  	if o.done {
  		return
  	}
  	if o.doing {
  		panic("nosync: Do called within f")
  	}
  	o.doing = true
  	defer func() {
  		o.doing = false
  		o.done = true
  	}()
  	f()
  }
docs: // Do calls the function f if and only if Do is being called for the
  // first time for this instance of Once. In other words, given
  // 	var once Once
  // if once.Do(f) is called multiple times, only the first call will invoke f,
  // even if f has a different value in each invocation.  A new instance of
  // Once is required for each function to execute.
  //
  // Do is intended for initialization that must be run exactly once.  Since f
  // is niladic, it may be necessary to use a function literal to capture the
  // arguments to a function to be invoked by Do:
  // 	config.once.Do(func() { config.init(filename) })
  //
  // If f causes Do to be called, it will panic.
  //
  // If f panics, Do considers it to have returned; future calls of Do return
  // without calling f.
  //