How to Work with the Stan Submodule in rstan Repo?

In rstan, stan repo is defined as a submodule. The first time we clone rstan repo, the source of stan will not be downloaded. So we need to execute the following git command. Here is more doc about cloning a project with submodules.

$ git submodule update --init

And later, we need to update the submodule with

$ git submodule update

The following are an example:

$ git clone https://github.com/stan-dev/rstan.git rstan.repo
Cloning into 'rstan.repo'...
remote: Counting objects: 4696, done.
remote: Compressing objects: 100% (1618/1618), done.
remote: Total 4696 (delta 2733), reused 4675 (delta 2715)
Receiving objects: 100% (4696/4696), 14.80 MiB | 563 KiB/s, done.
Resolving deltas: 100% (2733/2733), done.
$ cd rstan.repo/
$ git submodule update --init
Submodule 'stan' (https://github.com/stan-dev/stan.git) registered for path 'stan'
Cloning into 'stan'...
remote: Counting objects: 67778, done.
remote: Compressing objects: 100% (28561/28561), done.
remote: Total 67778 (delta 39283), reused 66614 (delta 38174)
Receiving objects: 100% (67778/67778), 63.97 MiB | 1.66 MiB/s, done.
Resolving deltas: 100% (39283/39283), done.
Submodule path 'stan': checked out '04cb63965acee7ce0d0c3b547f4be54e090c8d1c'
$ git submodule update