code: func Get(ctx context.Context) (clock Clock) {
  	if v := ctx.Value(&clockKey); v != nil {
  		if f, ok := v.(Factory); ok {
  			clock = f(ctx)
  		}
  	}
  	if clock == nil {
  		clock = GetSystemClock()
  	}
  	return
  }
docs: // Get returns the Clock set in the supplied Context, defaulting to
  // SystemClock() if none is set.