It is highly likely that Brian is a frog. It is almost certain that Gertrude is a sheep. There is a very good chance that Bernhard is green. It is improbable that if 'Gertrude is a sheep and Bernhard is green' then Daniel took the milk. There is almost no chance that if 'Brian is a frog and Bernhard is green' then John discarded the apple. It is certain that if 'Gertrude is a sheep and Bernhard is green' then Sandra left the milk.
We believe that either 'Gertrude is a sheep' or 'Bernhard is green' but not both.
It is probably not the case that either 'Gertrude is a sheep' or 'Bernhard is green' but not both.
We believe that either 'Gertrude is a sheep' or 'Bernhard is green' but not both.

  and(A,B) :- A,B.
  or(A,B) :- A;B.
  nand(A,B) :- not(and(A,B)).
  nor(A,B) :- not(or(A,B)).
  xor(A,B) :- or(A,B), nand(A,B).

  %hop0
  0.9::factA.
  0.95::factB.
  0.8::factC.
  %hop1
  0.1::factX:-and(factB,factC).
  0.02::factY:-and(factA,factC).
  1.0::factZ:-and(factB,factC).

  %hop2
  conclusion:-xor(factB,factC).

  query(conclusion).
  
Either 'Gertrude is a sheep' or 'Bernhard is green' but not both.