Jun 2018 clarify: Handling of union types (#266, #441, #464)

[Jesper Louis Andersen]

Handle these cases

[Jesper Louis Andersen]

• 266 says "Union types must be unique"
• 441 says "Thou must not declare empty unions"
• 464 says "We messed up the description, lets fix it"

So this means we need two test cases:

• Create an empty union and have it fail
• Create a union with the same type occurring twice and have it fail

[Jesper Louis Andersen]

This has now been handled on master.