code: public function set($value)
      {
          if ($value !== null) {
              $value = $this->check($value);
          }
  
          if ($value !== $this->value) {
              $this->value = $value;
              $this->notify();
          }
  
          return $this;
      }
docs: Set the value
  
  @param  mixed $value
  @return this