LaTeX: ヤバくない \text の実装

bxamstext.sty

%% bxamstext.sty

%% package declaration
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{bxamstext}[2015/12/19 v0.2]

%% preparation
\def\bxtt@pkgname{bxamstext}

%% load packages
\RequirePackage{calc}
\RequirePackage{zref}

%--------------------------------------- amstext

%% load amsmath, so taht:
%  - patches to some LaTeX commands are reverted
%  - switch 'firstchoice@' is annihilated
\def\bxtt@tmpa{% commands to be reverted
  \do\stepcounter \do\addtocounter
  \do\GenericInfo \do\GenericWarning \do\GenericError}
\@ifpackageloaded{amstext}{}{%else
  \def\do#1{\expandafter\let\csname$\string#1\endcsname#1%
  }\bxtt@tmpa
  \RequirePackage{amstext}% load
  % revert the patch
  \def\do#1{\expandafter\let\expandafter#1\csname$\string#1\endcsname%
    \expandafter\let\csname$\string#1\endcsname\@undefined
  }\bxtt@tmpa
}
\let\bxtt@tmpa\relax
% 'firstchoice@' is always true
\firstchoice@true
\let\firstchoice@true\relax
\let\firstchoice@false\relax

%--------------------------------------- new version of \text

%% variables
\newcount\bxtt@id

%% zref settings
\zref@newlist{bxtt}
\zref@newprop{bxtt@ms}[0]{0}
\zref@addprop{bxtt}{bxtt@ms}

%% redeine \text@
% (this is the main process '\text' in math mdoe.)
\def\text@#1{%{<text>}
  \begingroup
    \bxtt@gen@label
    \bxtt@text@out{\zref@extract{\bxtt@label}{bxtt@ms}}{#1}%
    \bxtt@inspect@ms
  \endgroup}
\def\bxtt@text@out#1#2{%
%\message{<\bxtt@label:#1>}%
  \ifcase#1\relax
    \setbox\z@\hbox{#2}%
    \@tempdimb\fontdimen6 \textfont2\relax
    \setlength\@tempdima{\wd\z@*\ratio{18\p@}{\@tempdimb}}%
    \edef\bxtt@tmpa{\strip@pt\@tempdima}%
    \hb@xt@\z@{\box\z@\hss}\mskip\bxtt@tmpa mu\relax
  \or \textdef@\displaystyle\f@size{#2}%
  \or \textdef@\textstyle\f@size{\firstchoice@false #2}%
  \or \textdef@\textstyle\sf@size{\firstchoice@false #2}%
  \or \textdef@\textstyle \ssf@size{\firstchoice@false #2}%
  \fi}

%% \bxtt@gen@label
% Sets \bxtt@label.
\def\bxtt@gen@label{%
  \global\advance\bxtt@id\@ne
  \edef\bxtt@label{bxtt:\the\bxtt@id}}

%% \bxtt@inspect@ms
\def\bxtt@inspect@ms{\mathchoice
  {\bxtt@inspect@ms@a{1}}%
  {\bxtt@inspect@ms@a{2}}%
  {\bxtt@inspect@ms@a{3}}%
  {\bxtt@inspect@ms@a{4}}}
\def\bxtt@inspect@ms@a#1{%
  \zref@setcurrent{bxtt@ms}{#1}%
  \zref@labelbylist{\bxtt@label}{bxtt}}

%--------------------------------------- all done
\endinput
%% EOF

test-bxamstext.tex

% XeLaTeX/LuaLaTeX document
\documentclass[a4paper]{article}
\usepackage{fontspec}
\newfontfamily\fIpxm{IPAexMincho}
\usepackage{bxamstext}% little noise
%\usepackage{amstext}% much noise!
\newcommand\☃{\text{{\fIpxm ☃}%
  \typeout{Snowman's here!}}}% noise
\begin{document}

We have: $S = \Bigl\{\☃^{\☃^{\☃}}\Bigr\}$.

\end{document}