# An error indicating that a username is syntactically invalid. class InvalidUserError < StandardError; end # An error indicating that a domain is syntactically invalid. class InvalidDomainError < StandardError; end # An error indicating that a user does not exist. class NonexistentUserError < StandardError; end # An error indicating that a domain does not exist. class NonexistentDomainError < StandardError; end # An error indicating that some user already exists. For example, if # one tries to rename a user and the destination user already exists. class UserAlreadyExistsError < StandardError; end