Q.lhs

> module Q where

I have a situtation where it would be extremely useful to limit
the self-recursion of a data type to only a subset of its
constructors. Consider the datatype T below:

> data T
>   = A integer
>   | B T float
>   | C T [float]
>   | OnlyBs T [T] -- list should be only of B's
>   | OnlyCs T [T] -- list should be only of C's

In the last two constructors I would prefer that the final list
be limited to being composed only of B's and C's, as per the
comment. I could provide explicit constructors that deny any
other construction but I wonder if there's a way to express
this in the type system?