Change Relation#replace to remove tuples, then insert new tuples
Change Relation#replace to remove tuples, then insert new tuples

* The original operation would return the correct relation, but it wouldn't be
  using the correct operands. If the other base relations are not equivalent
  then it could have unintended side effects.
* A future optimization is going to minimize work, but I wanted to start with
  the proper algorithm.

dkubb/axiom