fn distinct_core<R2: Ord+Abelian+'static+From<i8>>(&self) -> Collection<G, K, R2> {
        self.threshold_named("Distinct", |_,_| R2::from(1i8))
    }
Distinct for general integer differences.

This method allows `distinct` to produce collections whose difference
type is something other than an `isize` integer, for example perhaps an
`i32`.
differential-dataflow/src/operators/reduce.rs