COSET: a program for deriving and testing merohedral and pseudo-merohedral twin laws

COSET is a program written in ISO C99 with POSIX extensions which uses left coset decompositions to determine possible merohedral and pseudo-merohedral twin laws. In addition to a stand-alone program, the code may be compiled as a Python extension module. The program can create SHELXL instruction files which incorporate the appropriate TWIN and BASF instructions for the possible twin law(s). COSET may also be directed to execute a locally installed copy of the SHELXL binary executable to test the candidate twin laws in trial refinements. This facilitates the quick screening and assessment of possible twin laws.