[pazz]

In a NestedTree, an empty subtree takes no line (#77). ArrowTree decorates with its own tree's positions, though, and asked that tree for the next sibling, so a node followed only by an empty subtree was drawn with a T-shaped connector, and its descendants with a vertical bar, leading to nothing:

before      after
R           R
├─➤a        └─➤a
│  └─➤a1       └─➤a1

ArrowTree now asks _has_next_sibling, which skips siblings that are trees holding nothing but empty trees, at all four places that decide between ├ and └ or draw a vertical bar (first lines, spacers and continuation lines). Trees without nested empty subtrees are unaffected.

The emptiness check is a loop with an explicit stack, so deeply nested empty trees do not hit the recursion limit.

Tests: test_arrows_skip_empty_subtrees covers the connector, the bar on a grandchild's line and a continuation line, with an empty subtree and a subtree holding only an empty subtree as trailing siblings. It fails without the fix. test_arrows_skip_deeply_nested_empty_subtree decorates a node followed by an empty tree wrapped 1500 levels deep (decoration only, since NestedTree navigation still recurses once per nesting level, a known limit).

Not covered: CollapsibleArrowTree and CollapsibleIndentedTree still draw a collapse icon on a node whose only children are empty subtrees, since collapsible asks the wrapped tree whether the node is a leaf.

Review findings and resolution

review-correctness

• Recursion per nesting level in the emptiness check: fixed, it is now iterative, with a 1500-level regression test.
• Quadratic work when a node with many lines below it is followed by a long run of empty subtrees: left, with a ponytail: comment. Each check walks the run of empties, so the cost is lines times empties, but only in that shape; NestedTree sibling moves already walk such runs, and the walker's line cache keeps a line from being decorated again until a refresh. A cache in ArrowTree would go stale when the tree changes and has no hook to clear it.
• Reuse NestedTree's navigation instead: left, ArrowTree is wrapped by the NestedTree and cannot see it.
• Other decorators were not checked: IndentedTree draws nothing per sibling; the collapse icon case is listed under "Not covered".

review-design

• ArrowTree over a decorated tree is unverified: checked, ArrowTree(CollapsibleTree(...)) inside a NestedTree renders └─➤a, since DecoratedTree.__getitem__ returns the wrapped entry. No test added, as decorators do not stack (Document that decorators do not stack and CollapseMixin needs a wrapper #98).
• Point _is_empty_tree at NestedTree._sanitize_position: done in its docstring.
• A private helper in decoration.py that knows about nested trees: kept, as the reviewer recommends over a public predicate on Tree.

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