add derivation from ‘concat’, ‘of’, ‘zero’, and ‘reduce’

README.md

@@ -886,6 +886,16 @@ to implement certain methods then derive the remaining methods. Derivations:
     }
     ```
 
+  - [`filter`][] may be derived from [`concat`][], [`of`][], [`zero`][], and
+    [`reduce`][]:
+
+    ```js
+    function(pred) {
+      var F = this.constructor;
+      return this.reduce((f, x) => pred(x) ? f.concat(F.of(x)) : f, F.zero());
+    }
+    ```
+
 If a data type provides a method which *could* be derived, its behaviour must
 be equivalent to that of the derivation (or derivations).