\documentclass{article}
\usepackage{latexml}
\usepackage{amsmath}
\begin{document}
% prescripted['UNKNOWN']

% Failed to attach: inpreScripted:<matchrule:$arg[0]> addScripts
% 	at ( ( ( ( ( ( preScripted doubtArgs ) moreFactors ) ) moreTerms addExpressionModifier ) extendFormula ) moreFormulae )

% Parse rule tree: 
% 1 Formulae 
% 2  Formula moreFormulae  
% 3  ( Expression extendFormula ) moreFormulae  
% 4  (( SignedTerm moreTerms addExpressionModifier ) extendFormula ) moreFormulae  
% 5  ((( Term ) moreTerms addExpressionModifier ) extendFormula ) moreFormulae  
% 6  (((( Factor moreFactors )) moreTerms addExpressionModifier ) extendFormula ) moreFormulae  
% 7  ((((( preScripted doubtArgs ) moreFactors )) moreTerms addExpressionModifier ) extendFormula ) moreFormulae  
% lex  (((( )))) 

${}_2{}^3x$
\end{document}