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Sorry, messed up something. New version followws: \newbox\chemlowbox \def\chemlow#1% {\setbox\chemlowbox\hbox{{\switchtobodyfont[small]#1}}} \def\chemhigh#1% {\ifvoid\chemlowbox \high{{\switchtobodyfont[small]#1}}% \else \lohi[left]{\box\chemlowbox}{{\switchtobodyfont[small]#1}}\fi } \def\finishchem{\ifvoid\chemlowbox \else \low{\box\chemlowbox}\fi} \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}}}}% \catcode`\-=\active \uccode`\~=`\- \uppercase{\def~{--}}% \domolecule }% \def\domolecule#1{#1\finishchem\egroup} \starttext {\bf A test \variant[Caps] in small caps} \molecule{HSO_4^{-}} \molecule{H_2SO_4} {\bfa A story about \molecule{SF_6}} {\scx A story about \molecule{sf_6}} {\bsb A story about \molecule{SF_6}} \stoptext