checkCIF procedure

PLAT340

 The average s.u. for X-Y bonds is tested (named bond-precision). X-Y will  
 generally be C-C bonds, unless there are none. In the last case the s.u.'s of  
 the lowest element numbers are considered (excluding hydrogen). There are  
 three test ranges: one for structures with the largest element Z < 20, one 
 for the largest Z in the range 20 to 39 and one for structures with Z(max) 40  
 or higher (_340, _341 and _342 respectively).