add docs

README.md

@@ -137,21 +137,21 @@ These examples incubate dotSCAD and dotSCAD refactors these examples. See [examp
 - list
 	- [util/bsearch](https://openhome.cc/eGossip/OpenSCAD/lib3x-bsearch.html)
 	- [util/has](https://openhome.cc/eGossip/OpenSCAD/lib3x-has.html)
-	- `util/find_index`
+	- [util/find_index](https://openhome.cc/eGossip/OpenSCAD/lib3x-find_index.html)
 	- [util/dedup](https://openhome.cc/eGossip/OpenSCAD/lib3x-dedup.html)	
 	- [util/flat](https://openhome.cc/eGossip/OpenSCAD/lib3x-flat.html)
 	- [util/reverse](https://openhome.cc/eGossip/OpenSCAD/lib3x-reverse.html)
 	- [util/slice](https://openhome.cc/eGossip/OpenSCAD/lib3x-slice.html)
 	- [util/sort](https://openhome.cc/eGossip/OpenSCAD/lib3x-sort.html)
 	- [util/sum](https://openhome.cc/eGossip/OpenSCAD/lib3x-sum.html)
-	- `util/swap`
+	- [util/swap](https://openhome.cc/eGossip/OpenSCAD/lib3x-swap.html)
 	- [util/zip](https://openhome.cc/eGossip/OpenSCAD/lib3x-zip.html)
 	- [util/every](https://openhome.cc/eGossip/OpenSCAD/lib3x-every.html)
 	- [util/some](https://openhome.cc/eGossip/OpenSCAD/lib3x-some.html)
 - random
 	- [util/choose](https://openhome.cc/eGossip/OpenSCAD/lib3x-choose.html)
 	- [util/rand](https://openhome.cc/eGossip/OpenSCAD/lib3x-rand.html)
-	- `util/shuffle`
+	- [util/shuffle](https://openhome.cc/eGossip/OpenSCAD/lib3x-shuffle.html)
 - string
 	- [util/parse_number](https://openhome.cc/eGossip/OpenSCAD/lib3x-parse_number.html)
 	- [util/split_str](https://openhome.cc/eGossip/OpenSCAD/lib3x-split_str.html)

docs/lib3x-find_index.md

@@ -0,0 +1,16 @@
+# find_index
+
+Returns the index of the first element in the list that satisfies the testing function. If no element passed the test, it returns -1.
+
+**Since:** 3.0
+
+## Parameters
+
+- `lt` : The list.
+- `test` : the testing function.
+
+## Examples
+
+    use <util/find_index.scad>;
+    
+    assert(find_index([10, 20, 30, 40], function(e) e > 10) == 1);

docs/lib3x-shuffle.md

@@ -0,0 +1,16 @@
+# shuffle
+
+Randomizes the order of the elements of a list.
+
+**Since:** 3.0
+
+## Parameters
+
+- `lt` : The list to shuffle.
+- `seed` : Random seed value.
+
+## Examples
+
+    use <util/shuffle.scad>;
+    
+    echo(shuffle([1, 2, 3, 4]));

docs/lib3x-swap.md

@@ -0,0 +1,17 @@
+# swap
+
+Swaps two elements in a list.
+
+**Since:** 3.0
+
+## Parameters
+
+- `lt` : The list.
+- `i` : The index of an element.
+- `j` : The index of the other element
+
+## Examples
+
+    use <util/swap.scad>;
+    
+    assert(swap([10, 20, 30, 40], 1, 3) == [10, 40, 30, 20]);

src/util/swap.scad

@@ -1,3 +1,13 @@
+/**
+* swap.scad
+*
+* @copyright Justin Lin, 2021
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib3x-swap.html
+*
+**/ 
+
 function swap(lt, i, j) =
     i == j ? lt :
     let(