Validation rules can also be set by numeric array 

My experience is that passing associative array to form validation is messy and unreadable.
so instead

    array(
      'field' => 'password',
      'label' => 'Password',
      'rules' => 'required'
    )

you just do

    array('username', 'Username', 'required')

You can still set it the first way. So reverse compatibility is preserved.