It would be great if the "accept" option to droppable could accept a function instead of a jQuery selector.

That function should have the event and UI arguments (same as drop option).

I want to be able to write quite complex logic for knowing whether the droppable should accept the draggable. For example, when re-ordering a list (where droppables are list-items with a class of "gap", interspersed with the content list-items), I would like to allow ul.gap to accept my draggable, UNLESS the gap is an immediate sibling of where the draggable started.