There is almost no chance that John discarded the apple. There is almost no chance that Daniel took the milk. There is a better than even chance that Bernhard is a swan. It is improbable that if 'Daniel took the milk' or 'Bernhard is a swan' or both then Greg is yellow. There is a very good chance that if either 'John discarded the apple' or 'Bernhard is a swan' but not both then Brian is yellow. It is unlikely that if 'Bernhard is a swan and Daniel took the milk' then Julius is a frog.
It is certain that either 'Bernhard is a swan' or 'Greg is yellow' but not both.
Chances are about even that either 'Bernhard is a swan' or 'Greg is yellow' but not both.
It is certain that either 'Bernhard is a swan' or 'Greg is yellow' 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
  0.02::factA.
  0.02::factB.
  0.6::factC.
  %hop1
  0.1::factX:-or(factB,factC).
  0.8::factY:-xor(factA,factC).
  0.2::factZ:-and(factC,factB).

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

  query(conclusion).
  
Either 'Bernhard is a swan' or 'Greg is yellow' but not both.