diff --git a/intervalset.go b/intervalset.go
index 2af281b..b9371fb 100644
--- a/intervalset.go
+++ b/intervalset.go
@@ -59,36 +59,53 @@ func (r Interval[E]) Set() Set[E] {
 // a Set.
 type Set[E Elem[E]] []Interval[E]
 
-// Collect returns the set of elements that are in any of rs.
+// Collect returns the set of elements that are in any of s.
 //
-// Collect performs better if Intervals in rs are Low-sorted in ascending order.
-func Collect[E Elem[E]](rs ...Interval[E]) Set[E] {
-	var s Set[E]
+// Collect performs better if s are sorted in ascending order by the Low field.
+func Collect[E Elem[E]](s ...Interval[E]) Set[E] {
+	return CollectInto(nil, s...)
+}
+
+// CollectInto returns the set of elements that are in any of s, overwriting x.
+//
+// CollectInto performs better if s are sorted in ascending order by the Low
+// field.
+func CollectInto[E Elem[E]](x Set[E], s ...Interval[E]) Set[E] {
+	x = x[:0]
 
-	for _, r := range rs {
-		if len(s) == 0 {
+	for _, r := range s {
+		if len(x) == 0 {
 			if r.Low.Compare(r.High) < 0 {
-				s = append(s, r)
+				x = append(x, r)
 			}
 
 			continue
 		}
 
-		switch r1 := &s[len(s)-1]; {
+		switch r1 := &x[len(x)-1]; {
 		case r1.High.Compare(r.Low) < 0:
 			if r.Low.Compare(r.High) < 0 {
-				s = append(s, r)
+				x = append(x, r)
 			}
 		case r1.Low.Compare(r.Low) <= 0:
 			if r1.High.Compare(r.High) < 0 {
 				r1.High = r.High
 			}
 		default:
-			s.Add(r)
+			x.Add(r)
 		}
 	}
 
-	return s
+	return x
+}
+
+// Normalize modifies s to return a normalized set.
+// It works like Set[E](s) with normalization.
+//
+// Normalize performs better if s are sorted in ascending order by the Low
+// field.
+func Normalize[E Elem[E]](s []Interval[E]) Set[E] {
+	return CollectInto(s, s...)
 }
 
 // Add adds range [r.Low, r.High) into x.
diff --git a/intervalset_test.go b/intervalset_test.go
index 3fd1269..e0908dc 100644
--- a/intervalset_test.go
+++ b/intervalset_test.go
@@ -41,6 +41,10 @@ func TestCreation(t *testing.T) {
 			Collect(Range[E](5, 1), Range[E](3, 7), Range[E](11, 9), Range[E](5, 1)),
 			Set[E]{{3, 7}},
 		},
+		{
+			Normalize([]Interval[E]{{1, 7}, {5, 13}, {11, 17}}),
+			Set[E]{{1, 17}},
+		},
 	}
 
 	for i, c := range testCases {