code: public function set($key, $value = null, $group = null)
      {
          if ($key instanceof Attribute) {
              return $this->setInstance($key);
          }
  
          if ($this->has($key)) {
              $this->update($key, $value, $group);
          } else {
              $this->items[$key] = $this->newAttribute($key, $value, $group);
          }
  
          return $this;
      }
docs: Add or update attribute.
  
  @param  \Sofa\Eloquence\Metable\Attribute|string $key
  @param  mixed $value
  @return $this