答案1
\vbox{\halign{}}
对于这种特殊情况,您可以使用构造:
\def\macroni{\leavevmode\vbox{\offinterlineskip\ialign{\hfil##\hfil\cr
\vrule height.03em width.25em\cr\noalign{\kern.13em}\i\cr}}}
\let\macronori=\=
\def\=#1{\ifx\i#1\macroni\else\macronori{#1}\fi}
S\=a\={\i}