don't swap when i == j

src/experimental/note.md

@@ -22,6 +22,8 @@ New modules/functions
 - `util/spherical_coordinate`
 - `util/every`
 - `util/some`
+- `util/swap`?
+- `util/shuffle`?
 
 - delete `m_cumulate`
 - delete `trianglate`, use `tri_ear_clipping`?

src/util/swap.scad

@@ -1,4 +1,5 @@
 function swap(lt, i, j) =
+    i == j ? lt :
     let(
 	    leng = len(lt),
 		a = min([i, j]),