diff --git a/3or4/Interpreter/Self.hs b/3or4/Interpreter/Self.hs
index d00a826..474da97 100644
--- a/3or4/Interpreter/Self.hs
+++ b/3or4/Interpreter/Self.hs
@@ -37,6 +37,7 @@ elemExp =
 
 substBrExp :: Exp
 substBrExp =
+  subst (Variable "if") ifExp .
   subst (Variable "elem") elemExp $ parse
   "\\subst. \\var. \\b. case b of \
   \ { Branch(c,vs,e') -> if (elem var vs) Branch(c,vs,e') Branch(c,vs, subst e') \