Provide more examples

[stevana]

Some ideas:

• [x] http://clrnd.com.ar/posts/2017-04-21-the-water-jug-problem-in-hedgehog.html
• [x] https://github.com/Quviq/QuickCheckExamples
• [ ] https://github.com/tlaplus/Examples/tree/master/specifications
• [ ] http://lamport.azurewebsites.net/tla/book.html
• [x] Distributed system, e.g. distributed hash map, with (uncleanly) killed processes, skewed clocks, gc/io pauses (killall -s STOP/CONT), lost packets, timeouts, network partitions.
• [x] Circular buffer and ticket dispenser examples from "Testing the Hard Stuff and Staying Sane"
• [x] Webserver example (implement db with reference? a real db would be more interesting as then we would need to flush it between runs?)
• [ ] The steam boiler: http://www.informatik.uni-kiel.de/~procos/dag9523/dag9523.html. Also see "Formal Methods for Industrial Applications, Specifying and Programming the Steam Boiler Control" (1995) -- Jean-Raymond Abrial, Egon Börger, Hans Langmaack.
• [ ] Light control case study: http://www.jucs.org/jucs_6_7/capturing_requirements_by_abstract/Boerger_E.ps.gz
• [ ] Queue and union/find from "Testing monadic code with QuickCheck" paper
• [x] Process registry example, https://github.com/hedgehogqa/haskell-hedgehog/blob/master/hedgehog-example/test/Test/Example/Registry.hs
• [ ] Cache from http://propertesting.com/book_stateful_properties.html
• [x] Bookstore from http://propertesting.com/book_case_study_stateful_properties_with_a_bookstore.html
• [ ] An example where actions might fail.
• [ ] Reliable broadcast with fault injection

[stevana]

Die hard example was introduced in #33.

[stevana]

Union/find example was added in #82.

[stevana]

Ticket dispenser example was added in #95.

[stevana]

CRUD webserver example added in #145.

[stevana]

Circular buffers example was added in #151.

[stevana]

Process registry example was added in #282.

[stevana]

For an example of a distributed system, see: https://github.com/adjoint-io/raft/blob/master/test/QuickCheckStateMachine.hs.

[stevana]

Union/find was reintroduced in #286.

[stevana]

Bookstore example was introduced in #332.