code: private function resolveSet()
      {
          $rules = $this->set;
          $resolvedRules = [];
  
          // expand sets
          foreach ($rules as $name => $value) {
              if ('@' === $name[0]) {
                  if (!\is_bool($value)) {
                      throw new \UnexpectedValueException(sprintf('Nested rule set "%s" configuration must be a boolean.', $name));
                  }
  
                  $set = $this->resolveSubset($name, $value);
                  $resolvedRules = array_merge($resolvedRules, $set);
              } else {
                  $resolvedRules[$name] = $value;
              }
          }
  
          // filter out all resolvedRules that are off
          $resolvedRules = array_filter($resolvedRules);
  
          $this->rules = $resolvedRules;
  
          return $this;
      }
docs: Resolve input set into group of rules.
  
  @return $this