```
% pandoc -f markdown -t native
\begin{proof}
\newcommand{\x}{\left.\right.}
\x
\end{proof}

1234567890abcdefghi

[\*\a](x)
^D
[ RawBlock
  ( Format "tex" ) "\\begin{proof}\n\\newcommand{\\x}{\\left.\\right.}\n\\left.\\right.\n\\end{proof}"
, Para
  [ Str "1234567890abcdefghi" ]
, Para
  [ Link
    ( "", [], [] )
    [ Str "*", RawInline ( Format "tex" ) "\\a" ]
    ( "x", "" )
  ]
]
```