Block tree with add, delete subtree and render

A document is an ordered forest of blocks, each with an id and some text. Implement:

1. add(id, parent_id, text): add a block as the last child of its parent, or as the last top-level block when it has no parent.
2. delete(id): remove the block and everything nested under it.
3. render(): return the document in pre-order, one line per block showing its depth, id and text, with siblings kept in the order they were added.