code: public static function findOneUsingMixin($mixin): Schema
      {
          $schemas = self::findAllUsingMixin($mixin);
          if (1 !== count($schemas)) {
              throw new MoreThanOneMessageForMixin($mixin, $schemas);
          }
  
          return current($schemas);
      }
docs: Return the one schema expected to be using the provided mixin.
  
  @param Mixin|string $mixin Mixin or curie major
  
  @return Schema
  
  @throws MoreThanOneMessageForMixin
  @throws NoMessageForMixin