How to use the segment_split_merge_tool

There seems a pair of input event map in `src/neuroglancer/ui/segment_split_merge_tools.ts`

```js
const MERGE_SEGMENTS_INPUT_EVENT_MAP = EventActionMap.fromObject({
'at:shift?+mousedown0': {action: 'merge-segments'},
'at:shift?+mousedown2': {action: 'set-anchor'},
});

const SPLIT_SEGMENTS_INPUT_EVENT_MAP = EventActionMap.fromObject({
'at:shift?+mousedown0': {action: 'split-segments'},
'at:shift?+alt+mousedown0': {action: 'split-and-select-segments'},
'at:shift?+mousedown2': {action: 'set-anchor'},
});
```
When I add a segmentation layer `precomputed://gs://fafb-ffn1-20190805/segmentation` and follow the keyboard commands in the above code, but there doesn't seem to be any prompts with the response. How do I use this tool if I want to use it