writer.go
func (f *FileLogger) W(format string, v ...interface{}) {
	f.Warn(format, v...)
}
func (f *FileLogger) W(format string, v ...interface{}) {
	f.Warn(format, v...)
}
func ( f * FileLogger ) W ( format string , v ... interface { } ) { f . Warn ( format , v ... ) }
// same with Warn()
same with Warn ()