|  If you have A, then you have either B or C, but not both



A is a given fact
some-item1 is under consideration
some-item2 is under consideration and is not equal that-item1
-------------------------------------------------------------------------
given A, that-item1 can be derived provided the following are not derived
   that-item2


some-item1 is under consideration
some-item2 is under consideration
that-item2 is not equal that-item1
-------------------------------------------------------------
that-item2 is under consideration and is not equal that-item1


this-item is under consideration
================================
  B
  C


this-item is a given fact
=========================
  A