createTask(ident id, ident taskId, double maxPoints) {
  assume exists //exams/exam[id]; # implicitly true in OO implementations
  assume not exists //exams/exam[id]/tasks/task[taskId];
 
  assume maxPoints >= 0;
 
  insert //exams/exam[id]/tasks <task id=[taskId] maxPoints=[maxPoints] />;
}