// Return true to indicate that this thread changed
// the bit, false to indicate that someone else did.
// In either case, the requested bit is in the
// requested state some time during the period that
// this thread is executing this call. More importantly,
// if no other thread is executing an action to
// change the requested bit to a state other than
// the one that this thread is trying to set it to,
// then the the bit is in the expected state
// at exit from this method. However, rather than
// make such a strong assertion here, based on
// assuming such constrained use (which though true
// today, could change in the future to service some
// funky parallel algorithm), we encourage callers
// to do such verification, as and when appropriate.

bool BitMap::par_at_put(idx_t bit, bool value) {

  {- -------------------------------------------
  (1) BitMap::par_set_bit() または BitMap::par_clear_bit() を呼び出すだけ.
      ---------------------------------------- -}

      return value ? par_set_bit(bit) : par_clear_bit(bit);
    }