A huge (and extremely useful) collection of useful ISIS functions, written and maintained by the Remeis observatory (and certainly containing thousands of working hours by a variety of people) can be found here:
Please cite properly when used!
How to contribute to the GIT repository
Log into gitlab, visit http://www.sternwarte.uni-erlangen.de/gitlab/remeis and click on "Request Access".
How to investigate the source code of available functions
In order to look up any code of a function as part of the ISISscripts navigate to its GIT-repository. You can see this folder once you require the ISISscripts (which might be done inside your .isisrc), e.g.,
Note for Remeis users: the repository is managed by GitLab. A local clone can be found in /data/git/isisscripts.git
Then change into the source code directory, e.g.,
Now the tricky part: you need to find that file, where the function which source code you want to investigate is defined. If you're lucky, the file is named according to the function's name (with .sl suffix), so you can try
find . -iname "mc_sig.sl"
If this does not work, you can try to grep its function definition
grep -R "define mc_sig"