changeAttributes(ident id, ident groupId, string day, string time, string location, integer maxSize) {
  assume exists //exercises/exercise[id]/groups/group[groupId]; # implicitly true in OO implementations
 
  assume maxSize >= 0;
  assume count(groupId, //exercises/exercise[id]/students/student/group) <= maxSize;
 
  update //exercises/exercise[id]/groups/group[groupId]/day day;
  update //exercises/exercise[id]/groups/group[groupId]/time time;
  update //exercises/exercise[id]/groups/group[groupId]/location location;
  update //exercises/exercise[id]/groups/group[groupId]/maxSize maxSize;
} # roles: assistant[id]