It is probably not the case that Fred dropped the apple. There is almost no chance that Gertrude is a sheep. Chances are about even that Greg is gray. Chances are about even that if either 'Gertrude is a sheep' or 'Greg is gray' but not both then Bernhard is white. It is improbable that if 'Greg is gray' or 'Gertrude is a sheep' or both then Brian is a rhino. It is probably not the case that if 'Greg is gray and Fred dropped the apple' then Julius is a frog.
It is probably the case that either 'Bernhard is white' or 'Gertrude is a sheep' but not both.
It is probably not the case that either 'Bernhard is white' or 'Gertrude is a sheep' but not both.
It is probably the case that either 'Bernhard is white' or 'Gertrude is a sheep' 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
  0.25::factA.
  0.02::factB.
  0.5::factC.
  %hop1
  0.5::factX:-xor(factB,factC).
  0.1::factY:-or(factC,factB).
  0.25::factZ:-and(factC,factA).

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

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