doc-src/isar.sty

 \newcommand{\indexoutertoken}[1]{\index{#1 (syntax)|bold}}
 \newcommand{\indexouternonterm}[1]{\index{#1 (syntax)|bold}}
 \newcommand{\indexisarcmd}[1]{\index{#1 (command)|bold}}
 \newcommand{\indexisarmeth}[1]{\index{#1 (method)|bold}}
 \newcommand{\indexisaratt}[1]{\index{#1 (attribute)|bold}}
+\newcommand{\indexisarreg}[1]{\index{#1 (register)|bold}}
 
 \newcommand{\isarcmd}[1]{\isarkeyword{#1}}
 \newcommand{\isartrans}[2]{#1 \mathbin{\,\to\,} #2}
 \newcommand{\isarkeep}[1]{#1 \mathbin{\,\to\,} #1}
 \newcommand{\isarmeth}{method}