This is not right, if an onLeave function is provided it will not be called instead you should aim for the same approach as native DOM event where returning false form the supplied even will prevent the default action.

Something like this:
```
myOnEnter = (val) ->
  return if onEnter?(val) is false
  highlight.value(val)
  return
```

... later ...

```
sel.on('mouseover', myOnEnter)
```