LaTeX /NewDocumentCommand: 如何判断可选参数为空？

//documentclass{article}//usepackage{xparse}//NewDocumentCommand{//myplus}{O{} O{}}{//IfNoValueTF{#2}{//IfNoValueTF{#3}{//#1}{//#1+//#2}}{//IfNoValueTF{#3}{//#1+//#2}{//#1+//#2+//#3}}}//begin{document}//myplus[3][5][7] //end{document}