code: function composition(f) {
      return forAll(_.Int).satisfy(function(a) {
        function g(x) { return x * 2; }
        function h(x) { return x - 1; }
  
        function returnComposition(b) {
          return g(h(b));
        }
  
        return f(a).map(returnComposition).isEqual(f(a).map(h).map(g));
      });
    }
docs: /*
  2nd Law: Composition
  'a.map((b) => f(g(b))' is equivalent to 'a.map(f).map(g)'