jquery droppable accept

Question

Can anyone tell me how I can write a function in accept condition and then how does it finds out that what to accept and what not to accept. For example, I want to accept

Answer 1

I've figured out a solution that works based on the condition that no draggable should be placed in a droppable already occupied by another draggable:

$(".placement").droppable({
    accept: function(elm) {
        // only allow draggables to the placement if there's no other draggable 
        // in the droppable
        if (!$(this).attr('isbusy'))
            return true;
    },
    drop: function(event, ui) {
        $(this).attr('isbusy', 'yeap'); // fill key with something

        var draggable = $(ui.draggable[0]);
        // free the draggable's previous droppable 
        if (draggable.attr('droppable')) {
            $('#' + draggable.attr('droppable')).attr('isbusy', '');
        }

        // save the new draggable's droppable
        draggable.attr('droppable', $(this).attr('id'));
    },
});