This projection did not existed in GeoTools 2, which was the version at the time the code has been forked. The Cassini-Soldner projection has been added in a later GeoTools version and ported to Geotk project, but not to Apache SIS. For Apache SIS the projection has been rewritten from scratch using:
IOGP Publication 373-7-2. Geomatics Guidance Note number 7, part 2.
Coordinate Conversions and Transformations including Formulas.
Revised September 2019.
ยง3.2.2 Cassini-Soldner
Initial implementation in Apache SIS is a direct, non-optimized implementation with almost verbatim
copy of above formulas (those formulas are themselves copy of other publications, the oldest ones
taking their origin two centuries ago). This initial implementation can be seen at commit
b44873a08fd4c02c8092d97c9ab9d8d9964218c4.