provide \isabellestyle{itunderscore} (requires underscore.sty);

--- a/lib/texinputs/isabelle.sty	Tue May 03 21:07:24 2011 +0200
+++ b/lib/texinputs/isabelle.sty	Tue May 03 21:18:43 2011 +0200
@@ -186,6 +186,14 @@
 \def\isacharverbatimclose{\isamath{\rangle\!\rangle}}%
 }
 
+\newcommand{\isabellestyleitunderscore}{%
+%requires underscore.sty
+\underscoreon%
+\isabellestyleit%
+\def\isacharunderscore{\_}%
+\def\isacharunderscorekeyword{\_}%
+}
+
 \newcommand{\isabellestylesl}{%
 \isabellestyleit%
 \def\isastyle{\small\sl}%