Taco's \molecule fails in mkiv

[Mojca Miklavec <mojca.miklavec.lists@gmail.com>]

Hello,

I did not try to analyze how the following macros work (they are too
complex), but they somehow fail in mkiv. (I don't need the "F"
kerning, so that portion of code may go away, but the macro itself is
very very handy.)

All I want is to have a macro that "converts" _{...} into \low{...}
and ^{...} into \high{...} (or possibly enable using both at the same
time).

\newbox\chemlowbox
\def\chemlow#1%
  {\setbox\chemlowbox\hbox{{\switchtobodyfont[small]#1}}}

\def\chemhigh#1%
  {\ifvoid\chemlowbox \high{{\switchtobodyfont[small]#1}}%
   \else \/\lohi{\box\chemlowbox}{{\switchtobodyfont[small]#1}}\fi }

\def\finishchem%
   {\ifvoid\chemlowbox \else
     \iffluor \fluorfalse \kern-.1em \fi\low{\box\chemlowbox}\fi}

% for "kerning" after F
\newif\iffluor

\unexpanded\def\molecule%
  {\bgroup
   \catcode`\_=\active \uccode`\~=`\_ \uppercase{\let~\chemlow}%
   \catcode`\^=\active \uccode`\~=`\^ \uppercase{\let~\chemhigh}%
   \dostepwiserecurse {65}{90}{1}
      {\catcode \recurselevel = \active
       \uccode`\~=\recurselevel
       \uppercase{\edef~{\noexpand\finishchem
                         \rawcharacter{\recurselevel}}}}%
   \uccode `\~=`\F \uppercase{\def~{\finishchem F\fluortrue}}%
   \catcode`\-=\active \uccode`\~=`\- \uppercase{\def~{--}}%
   %\loggingall
   \domolecule }%

\def\domolecule#1%
  {\expandafter\scantokens\expandafter
        {\detokenize{#1\finishchem}}\egroup}

\starttext
\molecule{CO_2}
\stoptext

Thanks for any hints,
    Mojca
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