We need to add to the generators:

https://github.com/twitter/algebird/blob/e45d2021e9e1b91ad4397c27e41287309fc87d29/algebird-test/src/test/scala/com/twitter/algebird/Generators.scala#L27

We need to make a generator for each of the types of intervals, and then make the implicit one:

oneOf(UniverseGen, EmptyGen, LowerInclusive, LowerExclusive, UpperInclusive, UpperExclusive, Intersection)

See:
https://github.com/twitter/summingbird/blob/develop/summingbird-core/src/test/scala/com/twitter/summingbird/DependantsTests.scala#L34

or:

https://github.com/twitter/summingbird/blob/develop/summingbird-core/src/test/scala/com/twitter/summingbird/DependantsTests.scala#L34
search for: "tree"

for an example