Changes since 2.1:

Changes since 2.1:
  - doc.txt: credited Dave & Ryan for their help in designing the API.
  - doc.txt: clarified purpose of equality predicate
  - doc.txt: removed inconsistencies in type language
  - general: require MzScheme 301.7 or greater (for contract stuff)
  - doc.txt: added notes explaining data sharing in this library

Changes from 2.0 to 2.1:
  - added extend-unique, exn:env:shadow
  - fixed weaken-env; it now preserves the library's internal invariants