We believe that Mary dropped the milk. It is impossible that Gertrude is a cat. It is probably the case that John went to the kitchen. It is almost certain that if 'John went to the kitchen' or 'Gertrude is a cat' or both then Winona is a sheep. It is impossible that if 'Gertrude is a cat' or 'John went to the kitchen' or both then Lily is white. Chances are about even that if 'Mary dropped the milk and John went to the kitchen' then Bernhard is a swan.
It is almost certain that either 'Winona is a sheep' or 'Mary dropped the milk' but not both.
Chances are about even that either 'Winona is a sheep' or 'Mary dropped the milk' but not both.
It is almost certain that either 'Winona is a sheep' or 'Mary dropped the milk' 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
  0.75::factA.
  0.0::factB.
  0.7::factC.
  %hop1
  0.95::factX:-or(factC,factB).
  0.0::factY:-or(factB,factC).
  0.5::factZ:-and(factA,factC).

  %hop2
  conclusion:-xor(factX,factA).

  query(conclusion).
  
Either 'Winona is a sheep' or 'Mary dropped the milk' but not both.