reinstate identity law

README.md

@@ -332,7 +332,8 @@ A value which has a Group must provide an `invert` method. The
 ### Filterable
 
 1. `v.filter(x => p(x) && q(x))` is equivalent to `v.filter(p).filter(q)` (distributivity)
-2. `v.filter(x => false)` is equivalent to `w.filter(x => false)`
+2. `v.filter(x => true)` is equivalent to `v` (identity)
+3. `v.filter(x => false)` is equivalent to `w.filter(x => false)`
    if `v` and `w` are values of the same Filterable (annihilation)
 
 #### `filter` method

laws/filterable.js

@@ -7,7 +7,8 @@ const {filter} = require('..');
 ### Filterable
 
 1. `v.filter(x => p(x) && q(x))` is equivalent to `v.filter(p).filter(q)` (distributivity)
-2. `v.filter(x => false)` is equivalent to `w.filter(x => false)`
+2. `v.filter(x => true)` is equivalent to `v` (identity)
+3. `v.filter(x => false)` is equivalent to `w.filter(x => false)`
    if `v` and `w` are values of the same Filterable (annihilation)
 
 **/
@@ -18,10 +19,16 @@ const distributivity = eq => v => p => q => {
   return eq(a, b);
 };
 
+const identity = eq => v => {
+  const a = v[filter](x => true);
+  const b = v;
+  return eq(a, b);
+};
+
 const annihilation = eq => v => w => {
   const a = v[filter](x => false);
   const b = w[filter](x => false);
   return eq(a, b);
 };
 
-module.exports = {distributivity, annihilation};
+module.exports = {distributivity, identity, annihilation};