The precision-indicating merging R factor value Rpim,
              for merging all intensities in this resoution shell.
              Ref: Diederichs, K. & Karplus, P. A. (1997). Nature Struct.
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                   Weiss, M. S. & Hilgenfeld, R. (1997). J. Appl. Cryst.
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                   Weiss, M. S. (2001). J. Appl. Cryst. 34, 130-135.