code: func CdromDiffOperation(d *schema.ResourceDiff, c *govmomi.Client) error {
  	log.Printf("[DEBUG] CdromDiffOperation: Beginning diff validation")
  	cr := d.Get(subresourceTypeCdrom)
  	for ci, ce := range cr.([]interface{}) {
  		cm := ce.(map[string]interface{})
  		r := NewCdromSubresource(c, d, cm, nil, ci)
  		if err := r.ValidateDiff(); err != nil {
  			return fmt.Errorf("%s: %s", r.Addr(), err)
  		}
  	}
  	log.Printf("[DEBUG] CdromDiffOperation: Diff validation complete")
  	return nil
  }
docs: // CdromDiffOperation performs operations relevant to managing the
  // diff on cdrom sub-resources