diff --git a/algebra.sty b/algebra.sty
index 26559d1..0d7025a 100644
--- a/algebra.sty
+++ b/algebra.sty
@@ -58,3 +58,5 @@
 
 
 \def\existsone{\exists!}
+\def\defon#1{\upharpoonright_{#1}}
+\DeclareMathOperator{\Id}{Id}