pub fn update(&mut self, element: D, change: R) {
        if self.buffer.len() == self.buffer.capacity() {
            if !self.buffer.is_empty() {
                self.handle.send_batch(&mut self.buffer);
            }
            // TODO : This is a fairly arbitrary choice; should probably use `Context::default_size()` or such.
            self.buffer.reserve(1024);
        }
        self.buffer.push((element, self.time.clone(), change));
    }
Adds to the weight of an element in the collection.
differential-dataflow/src/input.rs