createSheet(ident uid, ident id, ident sheetId, double maxPoints) {
  assume exists //accounts/account[uid]/assistant[id];
 
  assume exists //exercises/exercise[id];
  assume not exists //exercises/exercise[id]/sheets/sheet[sheetId];
 
  assume maxPoints >= 0;
 
  insert //exercises/exercise[id]/sheets <sheet id=[sheetId] maxPoints=[maxPoints] />;
}