1/1: Building boom (boom.idr)
Error: While processing right hand side of with block in eraseVar. Can't solve constraint between: S (countGreater thr xs) and countGreater thr xs.

boom.idr:23:42--23:66
 19 | eraseVar : (thr : Int) -> (ctx : Vect n Int) -> Fin n -> Maybe (Fin (countGreater thr ctx))
 20 | eraseVar thr (x :: xs) j with (x .<=. thr)
 21 |   eraseVar thr (x :: xs) FZ | True = Nothing
 22 |   eraseVar thr (x :: xs) FZ | False = Just FZ
 23 |   eraseVar thr (x :: xs) (FS i) | True = FS <$> eraseVar thr xs i
                                               ^^^^^^^^^^^^^^^^^^^^^^^^