[also make sameSymAtRoot on TExp O(1)
Helmut Grohne <grohne@cs.uni-bonn.de>**20150331130037
 Ignore-this: 6dae246e31beace52ea59ba42bb41308
 
 Saves 2.5% runtime on the test suite.
] hunk ./src/Syntax/Expressions.hs 90
-    sameSymAtRoot t1@(TAppE _ _) t2@(TAppE _ _)         =
-        on (==) (head . unfoldTAppE) t1 t2
+    sameSymAtRoot t1@(TAppE f1 _) t2@(TAppE f2 _)       = sameSymAtRoot f1 f2