:: Upd   = Upd
:: Expr  = Expr
:: Stmt  = Stmt
:: MTask = MTask Int // String

class arith v where
  lit :: t -> v t Expr | ...
  (+.) infixl 6 :: (v t p) (v t q) -> v t Expr | ...
  ...
class boolExpr v where
  (&.) infixr 3 :: (v Bool p) (v Bool q) -> v Bool Expr | ...
  ...
  (==.) infix 4 :: (v a p) (v a q) -> v Bool Expr | ...
  ...