We doubt that Mary dropped the milk. It is almost certain that Bernhard is a swan. There is little chance that Julius is a lion. It is probably not the case that if either 'Mary dropped the milk' or 'Bernhard is a swan' but not both then Brian is green. Chances are about even that if either 'Julius is a lion' or 'Mary dropped the milk' but not both then Fred left the football. It is unlikely that if 'Mary dropped the milk and Bernhard is a swan' then Daniel took the milk.
We believe that either 'Bernhard is a swan' or 'Brian is green' but not both.
We believe that either 'Bernhard is a swan' or 'Brian is green' but not both.
It is impossible that either 'Bernhard is a swan' or 'Brian 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
  0.2::factA.
  0.95::factB.
  0.1::factC.
  %hop1
  0.25::factX:-xor(factA,factB).
  0.5::factY:-xor(factC,factA).
  0.2::factZ:-and(factA,factB).

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

  query(conclusion).
  
Either 'Bernhard is a swan' or 'Brian is green' but not both.