grantAssistantRights(ident username, ident exerciseId) {
  assume exists //accounts/account[username]; # implicitly true in OO implementations
  assume exists //exercises/exercise[exerciseId];
 
  assume not exists //accounts/account[username]/assistant[exerciseId]);
  assume not exists //exercises/exercise[exerciseId]/assistant[account]; # implied by integrity and the assumption before
 
  insert //accounts/account[username] <assistant exercise=[exerciseId] />;
  insert //exercises/exercise[exerciseId] <assistant account=[account] />;
}