add Filterable (fix #46)

docs/spec.md

@@ -130,6 +130,7 @@ equivalent to that of the derivation (or derivations).
 * [Group](#group)
 * [Semigroupoid](#semigroupoid)
 * [Category](#category)
+* [Filterable](#filterable)
 * [Functor](#functor)
 * [Bifunctor](#bifunctor)
 * [Contravariant](#contravariant)
@@ -250,6 +251,21 @@ support `Semigroupoid` algebra for the same `T`, and obey following laws:
   1. Left identity: `M.compose(M.id(), a) ≡ a`
 
 
+### Filterable
+
+```js
+Filterable<T> {
+  filter: <a>(a => boolean, T<a>) => T<a>
+}
+```
+
+Module must match the `Filterable` signature for some type `T`, and obey following laws:
+
+  1. Distributivity: `F.filter(x => f(x) && g(x), a) ≡ F.filter(g, F.filter(f, a))`
+  1. Identity: `F.filter(x => true, a) ≡ a`
+  1. Annihilation: `F.filter(x => false, a) ≡ F.filter(x => false, b)`
+
+
 ### Functor
 
 ```js