Function source_leaf_emptied_by_move 

pub fn source_leaf_emptied_by_move(
    tree: &LayoutNode,
    block_id: &str,
    target_block_id: &str,
) -> Option<String>

The node id of block_id’s leaf, when move_stack_member(tree, block_id, target_block_id, ..) would delete that leaf: block_id is its only member and target_block_id lives in a different leaf. None for every other move (reorder, multi-member source, missing blocks, self-target).