Vladislav Zavialov authored Mar 25, 2019 and Marge Bot committed Apr 25, 2019

Previously we defined   type PV = P,
this had the downside that if we wanted to change PV,
we would have to modify P as well.

Now PV is free to evolve independently from P.

The common operations addError, addFatalError, getBit, addAnnsAt,
were abstracted into a class called MonadP.