[vtkusers] Include data from a different file using the XML file format

[Shi Jin]

Hi there,
I am dumping my simulation data in the XML file format, specifically the pvtu and vtu files for unstructured grid.
Since the data are dumped at different time steps for the same grid, I think I can reuse the mesh information, i,e, the coordinates and connectivity sections. To do that, I need a way to specify in the vtu files to include from another file. 
I know this can be done in the legacy VTK format but cannot find any information from the documentations.
I am wondering if anyone can provide more information on this.
Thank you very much.
Shi
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Shi Jin, PhD