Traits expecting interfaces implicitly leads to expensive runtime checks

Hi, I think traits will lend themselves to frequent runtime checking against the type of class in which they were used. Consider a trait designed for use with an Iterator class; the first thing might be to check the type of $this. <?php trait InnerIteratorImpl { public function apply() { if(!$this instanceof Iterator) throw new UnexpectedValueException(get_class($this) . ' not an Iterator ..'); ... } } ?> do you not think checks like this will be common / messy / performance costly; is there something i'm missing? thx, -nathan