why use dict:incr() not dict:set() in reset() method

if I want to set the key to 0, i can't use `reset(key, 0)`. I need to get a value, and then `reset(key, value)` ?
i am not understand why use `dict:incr()` not `dict:set()` in reset() method
```
function _M:reset(key, number)
if not number then
return nil, "expect a number at #2"
end
return self.dict:incr(key, -number, number)
end
```

Start by reviewing the reset() method shown in the issue and the dictionary API it calls, especially dict:incr() versus dict:set(). Determine whether reset(key, 0) should be supported and verify the expected behavior for setting a counter to zero.

Written by the indexing model from the issue text.