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