final Attends userAttend = grouping.getStudentAttend(userStudent);
if (userAttend == null) {
throw new InvalidStudentNumberServiceException();
}
if (strategy.checkAlreadyEnroled(grouping, studentUsername)) {
throw new InvalidSituationServiceException();
}
StudentGroup newStudentGroup = grouping.readStudentGroupBy(groupNumber);
if (newStudentGroup != null) {
throw new FenixServiceException();