done:
  - primitives
  - structures: list, product, sum, ...
      can compose structures, e.g., create list of list of product of sum of ...
      set and dictionary implemented as purely functional red-black tree
        keys allowed: (anything with total ordering)
          - primitive values
          - structural values
          - dynamically typed values
          - types
  - generic types and type unification (inferencing)
  - recursive types

todo:
  - dynamically typed values: have 'Any' or 'Top' type?
  

  - 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

  - interfaces (type classes)