add restriction operator

src/math/mathop.pysty

@@ -193,3 +193,13 @@ __END_OPTIONS_X__
 
 \newcommand\ab{\text{ab}}
 \newcommand\abelianization{^{\ab}}
+
+
+%Taken from user egreg on
+%   https://tex.stackexchange.com/a/22255
+\newcommand\restriction[2]{{% we make the whole thing an ordinary symbol
+  \left.\kern-\nulldelimiterspace % automatically resize the bar with \right
+  #1 % the function
+  \vphantom{\big|} % pretend it's a little taller at normal size
+  \right|_{#2} % this is the delimiter
+}}