done:
  - everything is properly typed, up to the meta-circular level
  - primitives
  - structures: list, product, sum
      can compose structures, e.g., create list of list of product of sum of ...
      list type is specialized for ListOfByte to use Uint8Array
  - generic types and type inferencing

todo:
  - to support sets of slots, need comparison of slots
      => comparison of values
        => problem: how to compare transformed values? their inputs can come from different types

      (a) tedious: put in every value:
            - the type
            - a comparison function for that type
          then first compare types, if types match, compare values.
          could generalize this by writing a compare function on 'typed' values.

      (b) dirty: use 'magic' function that compares any JS value

  - typeclass mechanism

  - what about type links: they connect anything to its type... what is the type of 'anything'?