🩹 fix: Correct split implementation.

src/split.js

@@ -9,12 +9,16 @@ import _split from './_split.js';
  * @param {Node} z Node to split at.
  */
 export default function split(x, z) {
-	assert(x instanceof Node);
-	assert(z instanceof Node);
 	if (x === z) {
 		return [null, x];
 	}
 
+	assert(x instanceof Node);
+	if (z === null) {
+		return [x, null];
+	}
+
+	assert(z instanceof Node);
 	_split(z);
 	return [x, z];
 }

test/src/split.js

@@ -0,0 +1,47 @@
+import test from 'ava';
+
+import {list} from '@iterable-iterator/list';
+
+import {str} from './_fixtures.js';
+
+import {from, values, split} from '#module';
+
+const macro = test.macro({
+	exec(t, sequence, i) {
+		const x = from(sequence);
+		let y = x;
+		for (let k = 0; k < i; ++k) y = y.next;
+		const [s1, s2] = split(x, y);
+		if (x === y) {
+			t.is(s1, null);
+			t.is(s2, x);
+		} else {
+			t.is(s1, x);
+			t.is(s2, y);
+		}
+
+		const left = list(values(s1)).join('');
+		const right = list(values(s2)).join('');
+		const expected = [sequence.slice(0, i), sequence.slice(i)];
+		t.deepEqual([left, right], expected);
+	},
+	title(title, sequence, i) {
+		return title ?? `split(${str(sequence)}, ${i})`;
+	},
+});
+
+test(macro, '', 0);
+test(macro, 'a', 0);
+test(macro, 'a', 1);
+test(macro, 'ab', 0);
+test(macro, 'ab', 1);
+test(macro, 'ab', 2);
+test(macro, 'abc', 0);
+test(macro, 'abc', 1);
+test(macro, 'abc', 2);
+test(macro, 'abc', 3);
+test(macro, 'abcd', 0);
+test(macro, 'abcd', 1);
+test(macro, 'abcd', 2);
+test(macro, 'abcd', 3);
+test(macro, 'abcd', 4);