signOutGroup(ident uid, ident id, ident studentId) {
  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]/group; # implies existence of the exercise and student, even the group
 
  remove //exercises/exercise[id]/students/student[studentId]/group;
  update //exercises/exercise[id]/groups/group[//exercises/exercise[id]/students/student[studentId]/group]/curSize
    (//exercises/exercise[id]/groups/group[//exercises/exercise[id]/students/student[studentId]/group]/curSize - 1)
}