function generateSum(gens) {
  var len = gens.length;
  var result = generatorBless(function (size) {
    var idx = random(0, len - 1);
    return sum.addend(idx, len, gens[idx](size));
  });

  return utils.curried2(result, arguments);
}
- `generator.sum(gens: (generator a, generator b...)): generator (a | b...)`