1 \def\@mathbb#1{% 2 \ifx#1\@empty\let\@next\@empty\else\@doublestruck#1\let\@next\@mathbb\fi 3 \@next} 4 \newcommand{\mathbb}[1]{\@callprim{\@mathbb}{#1\@print{\@empty}}} 5 \DeclareSymbolHtml{\ulcorner}{X231C} 6 \DeclareSymbolHtml{\urcorner}{X231D} 7 \DeclareSymbolHtml{\llcorner}{X231E} 8 \DeclareSymbolHtml{\lrcorner}{X231F}