[pazz]

Shortens urwidtrees/tree.py without changing behaviour; found by an audit for over-engineering.

• Tree.next_position and prev_position return early instead of nesting, and prev_position returns parent_position(pos) directly instead of copying it through a None check.
• SimpleTree's private helpers _get_substructure and _get_node read self._treelist instead of taking it as an argument that was always self._treelist, and _confirm_pos is one line.

Navigation reads the tree's data through _get_node, never through __getitem__, so a subclass that overrides __getitem__ (to wrap or hide widgets) navigates as before. test_getitem_override covers that; it fails if _confirm_pos goes through self[pos].

Review findings and resolution

review-correctness

• _confirm_pos went through self[pos], so a SimpleTree subclass overriding __getitem__ changed navigation (raising or hiding positions): fixed, with a regression test.
• The private _get_node/_get_substructure signatures change: left; they are private and unused elsewhere.
• Checked without findings: the full suite on urwid 3.0.4 and 2.1.2; identical forward and reverse walks on a 1500-level chain and on 10000 children, with unchanged timing.

review-design

• The same __getitem__ coupling, with a subclass that never stopped walking: fixed, with the test asked for.
• parent_position and last_child_position keep the older style: left, to keep this change to what it states.

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