              A description of the extinction correction method applied with\
               the data item _refine.ls_extinction_coef. This description should\
               include information about the correction method, either\
               'Becker-Coppens' or 'Zachariasen'  The latter is sometimes\
               referred to as the 'Larson' method even though it employs\
               Zachariasen's formula.\
\
               The Becker-Coppens procedure is referred to as 'type 1' when\
               correcting secondary extinction dominated by the mosaic spread;\
               as 'type 2' when secondary extinction is dominated by particle\
               size and includes a primary extinction component; and as 'mixed'\
               when there is a mixture of types 1 and 2.\
\
               For the Becker-Coppens method it is also necessary to set the\
               mosaic distribution as either 'Gaussian' or 'Lorentzian'; and the\
               nature of the extinction as 'isotropic' or 'anisotropic'. Note\
               that if either the 'mixed' or 'anisotropic' corrections are\
               applied the multiple coefficients cannot be contained in\
               *_extinction_coef and must be listed in _refine.details.\
\
               Ref: Becker, P. J. \& Coppens, P. (1974). Acta Cryst. A30,\
                    129-153.\
                    Zachariasen, W. H. (1967). Acta Cryst. 23, 558- 564.\
                    Larson, A. C. (1967). Acta Cryst. 23, 664-665.