Missing documentation for pattern matching and extensible data types

[artemisSystem]

Currently, the documentation page just says TODO for these two topics (3.3.4 and 3.3.5). It would be nice to at least have a sample program showcasing these features, in lieu of full documentation. I'm just curious to experiment with extensible data types, and with a short example here to get me started, i might be able to write an example program or documentation myself.

[danny88o]

Might be worth having a look at this discussion #454