Add the rank-model extend concatenating two sources at the top level

node-graph/libraries/core-types/src/extent.rs

@@ -75,4 +75,10 @@ impl LevelIn {
 	pub fn pushed(&self) -> bool {
 		self.level + 1 == self.depth
 	}
+
+	/// Whether the query targets the topmost level; `pushed` under the name a
+	/// non-creator (concat, remap) reads naturally.
+	pub fn top(&self) -> bool {
+		self.pushed()
+	}
 }

node-graph/libraries/core-types/src/gpoll.rs

@@ -185,6 +185,16 @@ impl Extent {
 			_ => Extent::Free,
 		})
 	}
+
+	/// The sum of two extents, used to concatenate a level; a free operand
+	/// counts as one lane, so a scalar edge joins a concat as a single item.
+	pub fn sum(a: GPoll<Extent>, b: GPoll<Extent>) -> GPoll<Extent> {
+		let lanes = |extent| match extent {
+			Extent::Exactly(count) => count,
+			Extent::Free => 1,
+		};
+		a.zip(b).map(|(a, b)| Extent::Exactly(lanes(a) + lanes(b)))
+	}
 }
 
 /// A query over a node's nesting levels: one level, the product below or above

node-graph/libraries/core-types/src/node.rs

@@ -490,6 +490,15 @@ impl<'a, N> LazyInput<'a, N> {
 	{
 		self.cell.eval_input(self.input_index, self.node, ctx)
 	}
+
+	/// The edge's composite extent, for kernels that split or shift indices
+	/// over their sources.
+	pub fn extent<Input>(&self, ctx: &Input, at: Level) -> GPoll<Extent>
+	where
+		N: Node<Input>,
+	{
+		self.node.extent(ctx, at)
+	}
 }
 
 #[cfg(test)]