Fix group with correct group order

group.go

@@ -2,14 +2,19 @@ package goterators
 
 // Group groups elements into the nested level. Use a build-group function to define group type.
 func Group[T any, G comparable](source []T, buildGroup func(item T) G) [][]T {
+	groupOrder := make([]G, 0, len(source))
 	m := map[G][]T{}
 	for index, item := range source {
 		group := buildGroup(item)
+		_, existed := m[group]
+		if !existed {
+			groupOrder = append(groupOrder, group)
+		}
 		m[group] = append(m[group], source[index])
 	}
 
 	output := make([][]T, 0, len(m))
-	for key := range m {
+	for _, key := range groupOrder {
 		output = append(output, m[key])
 	}
 	return output