public void crabDownDirBlockForModification(BlockId blk){
  lockTbl.xLock(blk,txNum);
  writenIndexBlks.add(blk);
}

public void crabDownDirBlockForModification ( BlockId blk ) { lockTbl . xLock ( blk , txNum ) ; writenIndexBlks . add ( blk ) ; }
sets exclusive lock on the directory block when crabbing down for modification .
