public function each(Callable $f) : void {
        foreach ($this->items as $i) {
            $f($i);
        }
    }
public function each(Callable $f) : void {
        foreach ($this->items as $i) {
            $f($i);
        }
    }
public function each ( Callable $ f ) : void { foreach ( $ this -> items as $ i ) { $ f ( $ i ) ; } }
run a function against each item in the collection

@param callable $f
run a function against each item in the collection