[rather than returning a [] that happens to be a Subst, return a nullSubst
Helmut Grohne <grohne@cs.uni-bonn.de>**20140725140225
 Ignore-this: 571c1d098fd0cd7fc42306c795b592f6
] hunk ./src/Igor2/RuleDevelopment/Matching.hs 119
-                (\_e -> return [])
+                (\_e -> return nullSubst)