/* Assign rails to all rows that are still unassigned.
  */
  for(pRow=p->pLast; pRow; pRow=pRow->pPrev){
    int parentRid;

    if( pRow->iRail>=0 ){
      if( pRow->pChild==0 && !pRow->timeWarp ){
        if( omitDescenders || count_nonbranch_children(pRow->rid)==0 ){
          /* no-op */
        }else{
          riser_to_top(pRow);
        }
      }
      continue;
    }
    if( pRow->isDup ){
      continue;

        }
      }
    }
    mask = BIT(pRow->iRail);
    pRow->railInUse |= mask;
    if( pRow->pChild ){
      assignChildrenToRail(pRow);
    }else if( !pRow->isLeaf && !omitDescenders ){
      riser_to_top(pRow);
    }
    if( pParent ){
      for(pLoop=pParent->pPrev; pLoop && pLoop!=pRow; pLoop=pLoop->pPrev){
        pLoop->railInUse |= mask;
      }
    }