fn distinct_total_core<R2: Abelian+From<i8>+'static>(&self) -> Collection<G, K, R2> {
        self.threshold_total(|_,_| 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/threshold.rs