The Entities Tutorial, amped up with instances and brought to MPS 2021.1. For earlier versions of MPS, please see the maintenance/mps branch. For versions earlier than 2018.3, please see version history saying "Migrate to ...".
The entitites tutorial is an MPS example that shows a standard pattern for implementing non-trivial data structures with simple and complex types, where complex types can contain complex types again (but also simple types).
If you need this tutorial to work in a certain version, please have a look at the tags!