Prevent a drop at a particular item in the list.

I want to exclude some items from being dropped at.

`

`
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1. item1
`
`
2. item2
`
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3. item3
`
`
4. item4
`
`

`isValidTarget` allows only for checking if the whole target container is droppable but I can't use it for a particular item in the container.

How can I retrieve id of the target item to be dropped at in `isValidTarget` in order to use it for preventing a drop in my customized `isValidTarget` function.

A code example would be a great help.

Sorry, English is not my strong asset.