diff --git a/package.json b/package.json
index 8fc86ee..083df24 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "@aureooms/js-topological-sorting",
   "description": "Topological sorting for JavaScript",
-  "version": "0.1.0",
+  "version": "0.2.0",
   "license": "AGPL-3.0",
   "author": "aureooms",
   "homepage": "https://aureooms.github.io/js-topological-sorting",
diff --git a/src/freeVertices.js b/src/freeVertices.js
new file mode 100644
index 0000000..c784aa8
--- /dev/null
+++ b/src/freeVertices.js
@@ -0,0 +1,12 @@
+/**
+ * Compute the free vertices of the input graph.
+ *
+ * @param {Iterable<any>} graph - The input graph as a list of edges.
+ * @returns {Iterable<any>} The free vertices of the input graph.
+ */
+export default function freeVertices(graph) {
+	const vertices = new Set();
+	for (const [u] of graph) vertices.add(u);
+	for (const [, v] of graph) vertices.delete(v);
+	return vertices;
+}
diff --git a/src/index.js b/src/index.js
index ff71f27..fb95f8e 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,4 +1,6 @@
+import freeVertices from './freeVertices.js';
 import kahn1962 from './kahn1962.js';
 import sorted from './sorted.js';
+import subroutine from './subroutine.js';
 
-export {kahn1962, sorted};
+export {freeVertices, kahn1962, sorted, subroutine};
diff --git a/src/kahn1962.js b/src/kahn1962.js
index 978068e..577494c 100644
--- a/src/kahn1962.js
+++ b/src/kahn1962.js
@@ -1,5 +1,5 @@
 /**
- * Kahn's algorithm.
+ * Kahn's algorithm for topological sorting.
  *
  * See https://en.wikipedia.org/wiki/Topological_sorting#CITEREFKahn1962
  *
diff --git a/src/sorted.js b/src/sorted.js
index 202a7a9..ed4c0fb 100644
--- a/src/sorted.js
+++ b/src/sorted.js
@@ -1,7 +1,7 @@
 import Heap from '@aureooms/js-pairing-heap';
 import {EfficientlyInvertiblePairs as Pairs} from '@aureooms/js-pairs';
 
-import kahn1962 from './kahn1962.js';
+import subroutine from './subroutine.js';
 
 /**
  * Sort the vertices topologically breaking ties according to a given function.
@@ -12,15 +12,10 @@ import kahn1962 from './kahn1962.js';
  * @throws {Error} If the input graph contains a cycle.
  */
 export default function* sorted(edges, breakTies = undefined) {
-	const graph = Pairs.from(edges);
-
 	const queue = breakTies ? new Heap(breakTies) : [];
-	const freeVertices = new Set();
-	for (const [u] of graph) freeVertices.add(u);
-	for (const [, v] of graph) freeVertices.delete(v);
-	for (const u of freeVertices) queue.push(u);
+	const graph = Pairs.from(edges);
 
-	yield* kahn1962(queue, graph);
+	yield* subroutine(queue, graph);
 
 	if (graph.size > 0) {
 		throw new Error(
diff --git a/src/subroutine.js b/src/subroutine.js
new file mode 100644
index 0000000..205bd16
--- /dev/null
+++ b/src/subroutine.js
@@ -0,0 +1,14 @@
+import freeVertices from './freeVertices.js';
+import kahn1962 from './kahn1962.js';
+
+/**
+ * Sort the vertices topologically using a queue to order the free vertices.
+ *
+ * @param {any} queue - The queue that will be used.
+ * @param {any} graph - The input graph as a list of edges.
+ * @returns {Iterable<any>} The vertices sorted in topological order.
+ */
+export default function subroutine(queue, graph) {
+	for (const u of freeVertices(graph)) queue.push(u);
+	return kahn1962(queue, graph);
+}