clarified package name (actually both pxfonts and txfonts exist and have this font);

--- a/NEWS	Sun Mar 21 23:56:54 2021 +0100
+++ b/NEWS	Mon Mar 22 00:07:55 2021 +0100
@@ -877,8 +877,8 @@
 
 *** Document preparation ***
 
-* High-quality blackboard-bold symbols from font "txmia" (package
-"txfonts"), for A..Z.
+* High-quality blackboard-bold symbols for A..Z, from font "txmia"
+(package "pxfonts").
 
 * Document markers are formal comments of the form \<^marker>\<open>marker_body\<close> that
 are stripped from document output: the effect is to modify the semantic

--- a/lib/texinputs/isabelle.sty	Sun Mar 21 23:56:54 2021 +0100
+++ b/lib/texinputs/isabelle.sty	Mon Mar 22 00:07:55 2021 +0100
@@ -39,7 +39,7 @@
 \DeclareRobustCommand{\isactrlesup}{\egroup\egroup\endmath\egroup}
 \newcommand{\isactrlbold}[1]{{\bfseries\upshape\boldmath#1}}
 
-%blackboard-bold (requires font txmia from txfonts)
+%blackboard-bold (requires font txmia from pxfonts)
 \DeclareSymbolFont{bbbfont}{U}{txmia}{m}{it}
 \SetSymbolFont{bbbfont}{bold}{U}{txmia}{bx}{it}
 \DeclareMathSymbol{\bbbA}{\mathord}{bbbfont}{129}