best way to disable dropping onto a sortable?

best way to disable dropping onto a sortable?


I'm struggling with the best way to approach this one, could one of
you jQuery UI gurus suggest a solution?
I have 2 lists ... the user is expected to drag items from list 1 and
drop them onto list 2. List 2 then behaves as a sortable, so after
these items from list 1 are dropped, they can be sorted within list
2.
I have that much working already!
My issue is ... List 2 should ONLY allow a maximum of 10 items to be
dropped in it.
What is the best way to disable the dropping of items from list 1 onto
list 2 for this situation? I cant find any hooks for sortable() or
draggable() that allow me to disable a drop conditionally. There must
be a proper way to do this.
This is the code I am using now ... and its functional otherwise ...
$("#draggable > li").draggable({
    helper: 'clone',
    connectToSortable: '#sortable',
    cursor: 'move',
    distance: 20,
    opacity: .5,
    revert: 'invalid'
});
$("#sortable").sortable({
    handle: '.handle',
    placeholder : 'sortableHelper',
    forcePlaceholderSize: 'true',
    receive : function (e, ui) {
        console.log($('#sortable').sortable('serialize'));
}
});
ul id=draggable
li ...
/ul
ul id=sortable
li ...
/ul