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

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