[fix Unify instance for TExp to be compatible with AUnify
Helmut Grohne <grohne@cs.uni-bonn.de>**20150519081327
 Ignore-this: 8883fdf62941c69b18aa242b723b8fb5
 
 Consider two TExps. t1 is a wildcard and t2 is a variable. The lgg of those
 will result in a variable t3. Now we expect that match t1 t3 succeeds and that
 applying the resulting substitution on t3 gives something `equal` to t1.
 However, that match previously failed.
 
 Now the question arises whether match t2 t1 should succeed. Previously, it did
 with the empty substitution. Yet, it seems strange to allow replacing a
 variable (t2) with a wildcard (t1) on a rhs as this weakens the specification.
 
 Thus the wildcard position in match is reversed now.
] hunk ./src/Syntax/UnifyExp.hs 26
-    match s t@(TWildE i1 _)     = checkMatch s t >> return nullSubst
+    match s@(TWildE i1 _) t     = checkMatch s t >> return nullSubst