* setVar allows `Var a` to be set to any value, the value
  is not constrained to be of type `a`. This seems necessary
  to allow, eg, `setVar v (Output (cmd "read"))`. On other
  hand, it's subptimal when using setVar with a Arith; in this
  case `a` should be some Integral.

* Allow: test (n `TEqual` 42)

  Perhaps by making Test an instance of Num?