public function next()
 {
     $this->it->next();
     if ($this->currentLen !== 0) {
         --$this->currentLen;
     }
 }
 public function next()
 {
     $this->it1->next();
     $this->it2->next();
 }
 public function next()
 {
     $this->it->next();
 }
 public function next()
 {
     $this->it->next();
     --$this->numLeft;
 }