code: public void lockEdge(String childName, LockMode mode) {
      Preconditions.checkState(endsInInode());
      Preconditions.checkState(mLockMode == LockMode.READ);
  
      lockEdgeInternal(childName, mode);
      mLockMode = mode;
    }
docs: Locks an edge leading out of the last inode in the list.
  
  For example, if the lock list is [a, a->b, b], lockEdge(c) will add b->c to the list, resulting
  in [a, a->b, b, b->c].
  
  @param childName the child to lock
  @param mode the mode to lock in