Supporting both covariance and contravariance for a single type parameter

Possible Duplicate:
Covariance and Contravariance on the same type argument

You can declare a generic type parameter as covariant by using the out keyword:

interface ICovariant<out R>

You can declare a generic type parameter as contravariant by using the in keyword:

interface IContravariant<in R>

And you can also support both for different type parameters:

interface IVariant<out R, in A>

So why can't you suport both for a single type parameter?