Unlike Scheme where exactness is implemented as taints, the Python
implementation associated exactness with data types.  This created
inheritance issues (making an exact subclass of floats would result
in the subclass having both an explicit Exact registration and an
inherited Inexact registration).  This was a problem for the
decimal module which was designed to span both exact and inexact
arithmetic.  There was also a question of use cases and no examples
were found where ABCs for exactness could be used to improve code.
One other issue was having separate tags for both the affirmative
and negative cases.  This is at odds with the approach taken
elsewhere in the Python (i.e. we don't have an ABC both Hashable
and Unhashable).