code: func (c *ResourceConfig) CheckSet(keys []string) []error {
  	var errs []error
  
  	for _, k := range keys {
  		if !c.IsSet(k) {
  			errs = append(errs, fmt.Errorf("%s must be set", k))
  		}
  	}
  
  	return errs
  }
docs: // CheckSet checks that the given list of configuration keys is
  // properly set. If not, errors are returned for each unset key.
  //
  // This is useful to be called in the Validate method of a ResourceProvider.