A type-level representation of a non-negative rational multiple of an integer power of pi.
Each type in this kind can be exactly represented at the term level by a value of type ExactPi, provided that its denominator is non-zero.
Note that there are many representations of zero, and many representations of dividing by zero. These are not excluded because doing so introduces a lot of extra machinery. Play nice! Future versions may not include a representation for zero.
Of course there are also many representations of every value, because the numerator need not be comprime to the denominator. For many purposes it is not necessary to maintain the types in reduced form, they will be appropriately reduced when converted to terms.