checkCIF procedure

PLAT916

 The Hooft y Parameter is calculated independently from the Bijvoet 
 differences and should have a value similar (observing the s.u.'s) to that of  
 the Flack x Parameter. See: Hooft, R.W.W, Straver, L.H. & Spek,A.L. (2008).
 J. Appl, Cryst. 41, 96-103. Thompson,A.L. & Watkin, D.J. (2009). Tetrahedron:
 Asymmetry, doi:10.1016/j.tetasy.2009.02.025. Large differences may arise in
 cases where the Flack parameter was not determined with a BASF/TWIN
 refinement or with essentially centrosymmetric data. See: Flack, H.D.,
 Bernardinelli, G, Clemente, D.A., Linden, A. Spek, A.L. (2006) Acta Cryst.
 B62, 695-701.