signUpGroup(ident id, ident studentId, ident groupId) {
  assume exists //exercises/exercise[id]/students/student[studentId]; # implies the existence of the exercise
  assume exists //exercises/exercise[id]/groups/group[groupId]; # also implies it, but once is really enough
 
  assume not exists //exercises/exercise[id]/students/student[studentId]/group;
  assume count(groupId, //exercises/exercise[id]/students/student/group) < //exercises/exercise[id]/groups/group[groupId]/maxSize;
 
  insert //exercises/exercise[id]/students/student[studentId]/group groupId;
}