code: func RegisterExtensionCheck(name string, oid asn1.ObjectIdentifier, filter *Filter, f func(pkix.Extension, *certdata.Data) *errors.Errors) {
  	extMutex.Lock()
  	Extensions = append(Extensions, extensionCheck{name, oid, filter, f})
  	extMutex.Unlock()
  }
docs: // RegisterExtensionCheck adds a new check to Extensions