docs: @Override public final void lcompute() {
      if( _hi-_lo >= 2 ) { // Multi-key case: just divide-and-conquer to 1 key
        final int mid = (_lo+_hi)>>>1; // Mid-point
        assert _left == null && _rite == null;
        T l = clone();
        T r = clone();
        _left = l; l._reservedMem = 0;
        _rite = r; r._reservedMem = 0;
        _left._hi = mid;          // Reset mid-point
        _rite._lo = mid;          // Also set self mid-point
        setPendingCount(1);
        // Compute min. memory required to run the right branch in parallel.  Min
        // memory equals to the max memory used if the right branch will be
        // executed single threaded (but in parallel with our left branch).
        // Assuming all memory is kept in the tasks and it is halved by reduce
        // operation, the min memory is proportional to the depth of the right
        // subtree.
        long reqMem = (log2(_hi - mid)+3)*memOverheadPerChunk();
  
        if(!_runSingleThreaded && MemoryManager.tryReserveTaskMem(reqMem)){
          _reservedMem += reqMem;   // Remember the amount of reserved memory to free it later.
          _left.fork();             // Runs in another thread/FJ instance
        } else {
          _left.compute2();
        }
        _rite.compute2();         // Runs in THIS F/J thread
      } else {
        if( _hi > _lo ) {         // Single key?
          try {
            map(_keys[_lo]);      // Get it, run it locally
          } catch( RuntimeException re ) { // Catch user-map-thrown exceptions
            throw H2O.setDetailMessage(re,re.getMessage()+" while mapping key "+_keys[_lo]);
          } catch( AssertionError re ) { // Catch user-map-thrown exceptions
            throw H2O.setDetailMessage(re,re.getMessage()+" while mapping key "+_keys[_lo]);
          } catch( OutOfMemoryError re ) { // Catch user-map-thrown exceptions
            throw H2O.setDetailMessage(re,re.getMessage()+" while mapping key "+_keys[_lo]);
          }
        }
        tryComplete();            // And this task is complete
      }
    }