The java implementations of several key 'parts' such as Register, import Hack.Gates.* or explicitly Hack.Gates.BuiltInGate ...
however, this package/ dir is not also included in the source package.
I use the source of the java to self-educate on the packages' flow etc., and being able to access/ read the BuiltInGate.java especially, as it is at the crux of the built-up gates' changes.
FOUND IT not where I thought it was going to be; it's under the simulator source tree:
good to see the code flow all the way down to the root java classes.