0.6.0 (2012-02-17):
    - Removed the phantom type argument for Variables.
0.5.0 (2011-07-12):
    - Moved UnificationFailure to Control.Unification.Types
    - Renamed NonUnifiable to TermMismatch
    - Control.Unification: exposed fullprune, semiprune, occursIn
    - Control.Unification: added unifyOccurs, subsumes
    - Control.Unification: (re)added symbolic names for binary operators
0.4.0 (2011-07-07):
    - Removed heterogeneous unification, and rewrote practically everything.
    - Added semipruning instead of full pruning.
    - Added visited-sets instead of occurs-checks.
    - Added weightedness to path compression (a la union--find).
    - This is the version emailed for the 2011-07-07 talk at McMaster.
0.3.6 (2011-06-18):
    - Forked from the Dyna2 project.