Exception on unnecessary inequality

``` haskell
-- throws Exception: Cannot process EGBoolValue BoolConst True
model :: FDModel
model = exists $ \col -> do
  [a,b] <- colList col 2
  xsum col @= 2
  a @>= 0
  b @>= 1
  2 @<= 1 -- this causes the exception
  return col
```
