为什么下面的代码不起作用？
\documentclass{article}
\usepackage{expl3}
\ExplSyntaxOn \cs_new_protected:Npn
\mycommand #1#2#3{ \use_i_i:nnn {#1}{#2}{#3} }
\ExplSyntaxOff
\begin{document}
$\mycommand{ abc } { {def} } { ghi }$
\end{document}
错误：
Command Line: D:\ProgramFiles\MikTexNEW\miktex\bin\x64\xelatex.exe --interaction=errorstopmode --synctex=-1 "ideje31.tex"
Startup Folder: C:\Users\hynek0\Desktop\TH0dipl
This is XeTeX, Version 3.14159265-2.6-0.999991 (MiKTeX 2.9.7050 64-bit)
entering extended mode
(ideje31.tex
LaTeX2e <2018-12-01>
(D:\ProgramFiles\MikTexNEW2\tex/latex\base\article.cls
Document Class: article 2019/12/20 v1.4l Standard LaTeX document class
(D:\ProgramFiles\MikTexNEW2\tex/latex\base\size10.clo))
(D:\ProgramFiles\MikTexNEW2\tex/latex\l3kernel\expl3.sty
(D:\ProgramFiles\MikTexNEW2\tex/latex\l3kernel\expl3-code.tex
(D:\ProgramFiles\MikTexNEW2\tex/latex\l3kernel\l3deprecation.def))
(D:\ProgramFiles\MikTexNEW2\tex/latex\l3backend\l3backend-xdvipdfmx.def))
(ideje31.aux)
! Undefined control sequence.
\mycommand #1#2#3->\use_i_i:nnn
{#1}{#2}{#3}
l.8 $\mycommand{ abc } { {def} } { ghi }
$
?
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PDFLaTeX Compilation Report (Pages: 0)
Errors: 0 Warnings: 0 Bad Boxes: 0
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