signOutGroup(ident uid, ident id, ident studentId) {
  assume exists /exercise[id] && /exercise[id]/open && 
         exists /account[uid]/student && /account[uid]/student/id = studentId
      || exists /account[uid]/assistant[id];
 
  assume exists /exercise[id]/student[studentId]/group;
    # implies existence of the exercise and student, even the group
 
 
  update /exercise[id]/group[/exercise[id]/student[studentId]/group]/curSize
    (/exercise[id]/group[/exercise[id]/student[studentId]/group]/curSize - 1)
  remove /exercise[id]/student[studentId]/group;
}