deleteGroup(ident uid, ident id, ident groupId) {
  assume exists /account[uid]/assistant[id];
 
  assume exists /exercise[id]/group[groupId];
 
  assume size(/exercise[id]/group[groupId]/tutor) = 0;
  assume count(groupId, /exercise[id]/student/group)) = 0;
 
  remove /exercise[id]/group[groupId];
}