Going to need the iterator.

src/source.re.c

@@ -174,9 +174,8 @@ finally:
  adding a source, (probably fine.) */
 const struct source *source_lookup(const struct sources *const s,
 	const union line64 range) {
-	struct source *looked;
+	size_t idx;
 	assert(s);
-	looked = s->list.data + source_tree_left(&s->dates, range);
-	/* verify ... */
-	return looked;
+	idx = source_tree_left(&s->dates, range);
+	return s->list.data + idx;
 }