It is highly likely that Gertrude is a mouse. It is highly unlikely that Bernhard is a swan. It is highly unlikely that Yann is hungry. There is almost no chance that if either 'Yann is hungry' or 'Gertrude is a mouse' but not both then Greg is gray. We doubt that if either 'Yann is hungry' or 'Gertrude is a mouse' but not both then John discarded the milk. It is improbable that if 'Gertrude is a mouse' or 'Bernhard is a swan' or both then Brian is a frog.
It is probably not the case that either 'John discarded the milk' or 'Brian is a frog' but not both.
It is probably not the case that either 'John discarded the milk' or 'Brian is a frog' but not both.
It is almost certain that either 'John discarded the milk' or 'Brian is a frog' 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.
  0.05::factB.
  0.05::factC.
  %hop1
  0.02::factX:-xor(factC,factA).
  0.2::factY:-xor(factC,factA).
  0.1::factZ:-or(factA,factB).

  %hop2
  conclusion:-xor(factY,factZ).

  query(conclusion).
  
Either 'John discarded the milk' or 'Brian is a frog' but not both.