Valid pseudo selectors. The value indicates the argument type:
Type: array
protected $validPseudo = array( "pattern" => 0, "statement" => 0, "expression" => 0, "declaration" => 0, "first-child" => 0, "last-child" => 0, "nth-child" => 1, "nth-last-child" => 1, "has" => 2, "is" => 2, "not" => 2, );