docs: function flatten(array, i) {
    i = i || 0;
  
    if(i >= array.length)
      return array;
  
    if(isArray(array[i])) {
      return flatten(array.slice(0,i)
        .concat(array[i], array.slice(i+1)), i);
    }
    return flatten(array, i+1);
  }