interface A
{
    public function x();
}

interface B
{
    public function y();
}

interface C extends A, B
{
    // x() and y() should be here
}

abstract class Abstract
{
    public function foo(A $a) {
        $a->x();
    }
}

abstract class LessAbstract extends Abstract
{
    public function foo(C $c) {
        $c->x();
        $c->y();
    }
}