[WIP] Layering (static case)

rangetree.py

@@ -220,3 +220,43 @@ def traverse_all(self) -> Iterator[TreeNode[K, V]]:
             yield from self.left.traverse_leaves()
             yield self
             yield from self.right.traverse_leaves()
+
+
+@dataclass(frozen=True)
+class LayeredTreeNode(TreeNode, Generic[K, V]):
+    """A 2D layered range tree."""
+    keys: List[K] = None
+    dim1: int = None  # dimension main tree is built on
+    dim2: int = None  # dimension auxiliary arrays are built on
+
+    def __post_init__(self, _key: Optional[K]):
+        assert self.dim1 is not None
+        assert self.dim2 is not None
+        super(LayeredTreeNode, self).__post_init__(_key)
+
+        if _key is not None:
+            object.__setattr__(self, 'keys', [_key])
+        else:
+            assert not self.is_leaf
+            # TODO: insert pointers here
+
+    @classmethod
+    def create_internal(
+        cls,
+        left: LayeredTreeNode,
+        right: LayeredTreeNode,
+        value=None
+    ) -> LayeredTreeNode[K]:
+        return cls(
+            is_leaf=False,
+            left=left,
+            right=right,
+            value=value,
+            size=left.size + right.size,
+            dim1=left.dim1,
+            dim2=left.dim2
+        )
+
+    # TODO: ideally, most of the fractional cascading magic would happen
+    # in __post_init__, etc.---we (hopefully) shouldn't have to change the
+    # create_* functions much.