revokeAssistantRights(ident username, ident exerciseId) {
  assume exists //accounts/account[username]/exercise[exerciseId]; # implies the account exists, which is implicitly true anyway
  assume exists //exercises/exercise[exerciseId]/assistant[account]; # implied by integrity and the assumption before
 
  remove //accounts/account[username]/exercise[exerciseId];
  remove //exercises/exercise[exerciseId]/assistant[account];
}