simple

var (
	// ErrUnknownEntityType indicates no workflow definition exists for the requested entity.
	ErrUnknownEntityType = errors.New("workflow: entity type not registered")
	// ErrInvalidTransition indicates the requested transition is not allowed.
	ErrInvalidTransition = errors.New("workflow: transition not allowed")
	// ErrMissingTransition indicates neither a transition name nor target state were supplied.
	ErrMissingTransition = errors.New("workflow: transition name or target state required")
	// ErrNilEntityID signals input validation failure.
	ErrNilEntityID = errors.New("workflow: entity id required")
)