Is there any way to change the output file name with VEX in Terrain Mesh ROP?

[watsonsong]

I want to export my terrain using the Terrain Mesh ROP, but I crop a sub set of my whole terrain and export them for performance. So the output index I want to fix by using the VEX, adjust the row and column index of the output file name. BTW, the extrude base will produce more triangle under the bottom, I delete them using a delete node. This make I can not expot the mesh using the output method on the Terrain Mesh ROP.