add doc

README.md

@@ -398,6 +398,12 @@ These examples incubate dotSCAD and dotSCAD refactors these examples. See [examp
 [**util/binary_search**(sorted, target[, lo, hi])](https://openhome.cc/eGossip/OpenSCAD/lib3x-binary_search.html) | search a value in a sorted list.
 [**util/count**(lt, test)](https://openhome.cc/eGossip/OpenSCAD/lib3x-count.html) | return the number of times `test` return `true` in the list.
 
+## Matrix
+
+ Signature | Description
+--|--
+[**matrix/m_replace**(m, i, j, value)](https://openhome.cc/eGossip/OpenSCAD/lib3x-m_replace.html) | replace the aᵢⱼ element of a matrix.
+
 ## Point Picking
 
  Signature | Description

docs/lib3x-m_replace.md

@@ -0,0 +1,32 @@
+# m_replace
+
+It replaces the aᵢⱼ element of a matrix.
+
+**Since:** 3.3
+
+## Parameters
+
+- `m` : A matrix.
+- `i` : The i-th row.
+- `j` : The j-th column.
+- `value` : The new value.
+
+## Examples
+
+    use <matrix/m_replace.scad>
+
+    original_m = [
+        [1, 2, 3, 4],
+        [5, 6, 7, 8],
+        [9, 10, 11, 12]
+    ];
+
+    expected = [
+        [1, 2, 3, 4],
+        [5, 6, 70, 8],
+        [9, 10, 11, 12]
+    ];
+
+    actual = m_replace(original_m, 1, 2, 70);
+
+    assert(actual == expected);