Origin for each line in src/mat.red from check-in c1ddb4c814:

c1ddb4c814 2021-03-01 jeff@gridfini: % example for matrix inversion
c1ddb4c814 2021-03-01 jeff@gridfini: mm:=mat ((1, 19, 9, 5),
c1ddb4c814 2021-03-01 jeff@gridfini: (5, 17, 7, 11),
c1ddb4c814 2021-03-01 jeff@gridfini: (9, 13, 11, 17),
c1ddb4c814 2021-03-01 jeff@gridfini: (13, 7, 15, 19));
c1ddb4c814 2021-03-01 jeff@gridfini: mminv:=1/mm;
c1ddb4c814 2021-03-01 jeff@gridfini: end;