Grant Assistant Rights

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