Document and enforce unsupported << for range trees

Add README and TREE_TYPES notes clarifying << support matrix across tree types.\n\nExplicitly document that FenwickTree and SegmentTree do not support << and require indexed update semantics.\n\nAdd unit and RSpec coverage asserting that << is unavailable and raises NoMethodError for these classes.

Author: evolve75

CHANGELOG.md

@@ -12,6 +12,8 @@ Changes section to scan for breaking or behavioral changes.
   instead of generic child-node attachment semantics.
 * Add `<<` insertion shorthand to `Tree::AATree` and `Tree::BTree` for
   `[key, value]`, `{ key:, value: }`, and `Entry` inputs.
+* Clarify in docs that `Tree::FenwickTree` and `Tree::SegmentTree` do not
+  support `<<`, and add tests asserting that unsupported behavior.
 
 ### 3.0.0pre / 2026-02-09
 

README.md

@@ -52,6 +52,16 @@ Available tree types include:
 * [Order-Statistic Tree][] under `Tree::OrderStatisticTreeNode` (require
   `tree/orderstatistictree`).
 
+Insertion operator (`<<`) support:
+
+* Supported:
+  `Tree::TreeNode` and TreeNode-derived node types,
+  `Tree::AATree`, `Tree::BTree`.
+* Not supported:
+  `Tree::FenwickTree`, `Tree::SegmentTree`.
+  These classes use explicit indexed updates (`update`, `[]=`) rather than
+  append-style insertion semantics.
+
 See [TREE_TYPES](./TREE_TYPES.md) for detailed descriptions, ASCII diagrams,
 and use cases for each tree type.
 

TREE_TYPES.md

@@ -129,6 +129,14 @@ conventions, while array-backed trees (Fenwick, Segment) and multi-entry trees
 These non-TreeNode types provide a TreeNode-like subset where it fits their
 semantics (Enumerable iteration, Comparable, and serialization).
 
+`<<` insertion support:
+
+- Supported:
+  `Tree::TreeNode` and TreeNode-derived node types, `Tree::AATree`,
+  `Tree::BTree`.
+- Not supported:
+  `Tree::FenwickTree`, `Tree::SegmentTree` (use `update` / `[]=`).
+
 ## Quick Examples
 
 ```ruby

spec/tree/fenwick_tree_spec.rb

@@ -100,4 +100,16 @@ def build_tree(values)
       expect(left < right).to be(true)
     end
   end
+
+  describe '<<' do
+    it 'is not supported' do
+      tree = build_tree([1, 2, 3])
+      expect(tree.respond_to?(:<<)).to be(false)
+    end
+
+    it 'raises when called' do
+      tree = build_tree([1, 2, 3])
+      expect { tree << 4 }.to raise_error(NoMethodError)
+    end
+  end
 end

spec/tree/segment_tree_spec.rb

@@ -93,4 +93,16 @@ def build_tree(values)
       expect(left < right).to be(true)
     end
   end
+
+  describe '<<' do
+    it 'is not supported' do
+      tree = build_tree([1, 2, 3])
+      expect(tree.respond_to?(:<<)).to be(false)
+    end
+
+    it 'raises when called' do
+      tree = build_tree([1, 2, 3])
+      expect { tree << 4 }.to raise_error(NoMethodError)
+    end
+  end
 end

test/test_fenwicktree.rb

@@ -120,5 +120,12 @@ def test_comparable
 
       assert_equal(-1, left <=> right)
     end
+
+    def test_shovel_operator_not_supported
+      tree = build_tree([1, 2, 3])
+
+      assert_equal(false, tree.respond_to?(:<<))
+      assert_raise(NoMethodError) { tree << 4 }
+    end
   end
 end

test/test_segmenttree.rb

@@ -114,5 +114,12 @@ def test_comparable
 
       assert_equal(-1, left <=> right)
     end
+
+    def test_shovel_operator_not_supported
+      tree = build_tree([1, 2, 3])
+
+      assert_equal(false, tree.respond_to?(:<<))
+      assert_raise(NoMethodError) { tree << 4 }
+    end
   end
 end