code: boolean isTask(int sequence) {
          int index = toIndex(sequence);
          checkSequence(index, sequence);
          return isTask[index];
      }
docs: Check if the sequence corresponds to a task
  
  @param sequence The sequence
  @return <tt>true</tt> if the sequence corresponds to a task, <tt>false</tt> otherwise
  @throws StaleTaskIdException if the solt overwritten