fn threshold<R2: Ord+Abelian+'static, F: FnMut(&K, &R1)->R2+'static>(&self, thresh: F) -> Collection<G, K, R2> {
        self.threshold_named("Threshold", thresh)
    }
Transforms the multiplicity of records.

The `threshold` function is obliged to map `R1::zero` to `R2::zero`, or at
least the computation may behave as if it does. Otherwise, the transformation
can be nearly arbitrary: the code does not assume any properties of `threshold`.

# Examples

```
use differential_dataflow::input::Input;
use differential_dataflow::operators::Threshold;

::timely::example(|scope| {
// report at most one of each key.
scope.new_collection_from(1 .. 10).1
.map(|x| x / 3)
.threshold(|_,c| c % 2);
});
```
differential-dataflow/src/operators/reduce.rs