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-# Design — four additive enhancements to `tree-structure` (target 4.1.0)
-
-Date: 2026-06-07. Source issues: #34, #35, #36, #33. Integration: **four separate PRs**,
-one per issue (`Closes #NN`), matching the repo's per-issue PR convention (#32).
-
-## Shared rules (every issue)
-
-- `explicitApi()` is on everywhere → every new public declaration is `public` with KDoc.
-- New tests live in `commonTest`, use `kotlin("test")`, and mirror the existing test style
-  (`TreeNodeV4Test.kt`, `TreeNodeNavigationTest.kt`, …).
-- Add a bullet to `CHANGELOG.md` under `## [Unreleased]` → `### Added` (do **not** bump
-  `PUBLISH_VERSION`; releases are a separate manual step).
-- Regenerate the binary-compatibility baseline with `./gradlew apiDump`; verify with `apiCheck`.
-- Do not modify the core public surface in a breaking way — these are additive only.
-
-## #34 — Structural mutation helpers → **members on `TreeNode`**
-
-Members (not extensions) because they need the private `_children`/`_parent` and the cycle/parent
-validation.
-
-- Extract the existing inline validation from `addChild` into a private
-  `validateAttachable(child)` and reuse it.
-- `insertChild(index: Int, child)` — validated insert at index (bounds `0.._children.size`).
-- `removeChildAt(index: Int): TreeNode<T>` — remove and return the detached child.
-- `replaceChild(index: Int, child): TreeNode<T>` — swap, return the old (now detached) child.
-- `moveChild(child, toIndex): Boolean` — reorder an existing direct child; no re-parent/cycle
-  check needed (it is already a child); `false` if `child` is not a direct child.
-- `addChildren(vararg children)` — validated append of several (per-child `addChild` semantics).
-- `sortChildren(comparator: Comparator<TreeNode<T>>)` — stable in-place reorder.
-
-## #35 — Query algorithms → **extensions** in new `TreeNodeQueryExt.kt`
-
-Built on the public API (`ancestors()`, `parent`, `depth()`, sequences). Return `null` when the
-two nodes are unrelated (different trees).
-
-- `lowestCommonAncestor(other): TreeNode<T>?` — deepest common node; includes self/other as
-  candidates (`LCA(a, a) == a`, `LCA(a, descendantOfA) == a`).
-- `distance(other): Int?` — `depth(this) + depth(other) - 2 * depth(lca)`.
-- `pathBetween(other): List<TreeNode<T>>?` — `[this … lca … other]`.
-- `contains(value): Boolean` — value search over the subtree, including the receiver.
-
-Document complexity (parent-walk based; O(depth) for LCA/distance, O(n) for `contains`).
-
-## #36 — Customizable `prettyString()` → **extension** in new `TreeNodePrettyPrintExt.kt`
-
-- `data class TreeConnectors(branch, lastBranch, vertical, empty)` with a `companion`:
-  `Default` (current Unicode box-drawing) and `Ascii`.
-- `fun TreeNode<T>.prettyString(connectors = TreeConnectors.Default,
-  render: (value: T, depth: Int, isLast: Boolean) -> String = { v, _, _ -> v.toString() }): String`.
-- The no-arg member `prettyString()` is unchanged; member resolution wins for `node.prettyString()`,
-  so existing behaviour and output are byte-for-byte identical.
-
-## #33 — Immutable variant → **new module `:tree-structure-immutable`**
-
-Largest item; fully isolated from the other three except for `settings.gradle.kts`,
-`libs.versions.toml`, the root `build.gradle.kts` Dokka aggregation block, and `CHANGELOG.md`.
-
-- `settings.gradle.kts`: `include(":tree-structure-immutable")`.
-- New `build.gradle.kts` mirroring the serialization module's KMP target matrix; deps
-  `api(project(":"))` + `kotlinx-collections-immutable`.
-- `libs.versions.toml`: add `kotlinx-collections-immutable` (a recent stable, e.g. 0.3.8) and
-  wire it.
-- Root `build.gradle.kts`: add `dokka(project(":tree-structure-immutable"))` to the aggregation.
-- `ImmutableTreeNode<T>` backed by `persistentListOf` children; `addChild` / `removeChild` /
-  `mapValues` return a new root with unchanged subtrees structurally shared. Mirror pre/post/
-  level-order traversal helpers.
-- Its own `tree-structure-immutable/api/` baseline via `apiDump`.
-
-## Orchestration
-
-- Four git worktrees off `origin/master`, one branch each (`feat/...`), for full isolation — no
-  collisions on `TreeNode.kt`, `api/*.api`, or `CHANGELOG.md`.
-- A workflow pipelines each issue: implement (code + KDoc + tests + CHANGELOG + `apiDump`) and
-  verify locally (`jvmTest` + `apiCheck`), then an adversarial reviewer checks issue-conformance,
-  test quality, invariants, explicit API, `.api` freshness, and that it builds. Local checks are
-  JVM-only for speed; CI validates the full KMP matrix on each PR.
-- Then push each passing branch and open a PR (`Closes #NN`); remove the worktrees afterward.