signUpGroup(ident uid, ident id, ident studentId, ident groupId) {
  assume exists //exercises/exercise[id] && //exercises/exercise[id]/open && 
         exists //accounts/account[uid]/student && //accounts/account[uid]/student/id = studentId
      || exists //accounts/account[uid]/assistant[id];
 
  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 //exercises/exercise[id]/groups/group[groupId]/curSize < //exercises/exercise[id]/groups/group[groupId]/maxSize;
 
  insert //exercises/exercise[id]/students/student[studentId]/group groupId;
  update //exercises/exercise[id]/groups/group[groupId]/curSize (//exercises/exercise[id]/groups/group[groupId]/curSize + 1);
}