[don't assign unused variables on node deconstruction when generating C code.
John Meacham <john@repetae.net>**20060129025336] hunk ./C/FromGrin.hs 311
+        fve = freeVars e'
hunk ./C/FromGrin.hs 313
-    let ass = [assign  a (project (arg i) tmp) | a <- as' | i <- [( 1 :: Int) ..] ]
+    let ass = concat [perhapsM (a `Set.member` fve) $ assign  a' (project (arg i) tmp) | a' <- as' | Var a _ <- as |  i <- [( 1 :: Int) ..] ]