The stan version is in form of
first version is 1.0.0, indicating major version 1, minor version
0 and patch level 0. Functionality only changes with minor versions
and backward compatibility will only be affected by major versions.
A character string giving the version of Stan used in this version of RStan.
The Stan Development Team Stan Modeling Language User's Guide and Reference Manual. http://mc-stan.org/.
stan_version()#>  "2.21.0"