1) Write a Hindley Milner type checker for that injective datatype recursor language

https://wikimedia.org/api/rest_v1/media/math/render/svg/431b94815103ac9dc77d0e92739456c3c2c90803
https://github.com/sdiehl/write-you-a-haskell/blob/master/chapter7/poly_constraints/src/Infer.hs
https://github.com/sdiehl/write-you-a-haskell/blob/master/chapter7/poly/src/Infer.hs

2) Extend it to allow for referencing declerations "further down" in file, but make sure it
terminates by forbidding corecursion (simply done by building a reference graph and then looking at it)

3) Add Kind env and use it to have "type of types"

4) Type annotated tree?

5) Make SECD machine good enough to execute it

6) Write backend for SECD machine code