code: public function getMixin($mixinId)
      {
          if (isset($this->mixins[$mixinId])) {
              return $this->mixins[$mixinId];
          }
  
          if (isset($this->mixinsByCurie[$mixinId])) {
              return $this->mixinsByCurie[$mixinId];
          }
  
          throw new MixinNotDefined($this, $mixinId);
      }
docs: @param string $mixinId
  
  @return Mixin
  @throws MixinNotDefined