It is improbable that Gertrude is a sheep. It is highly likely that Brian is yellow. It is unlikely that Julius is a lion. There is little chance that if 'Gertrude is a sheep' or 'Julius is a lion' or both then Bernhard is a swan. There is little chance that if 'Gertrude is a sheep and Julius is a lion' then Greg is gray. It is unlikely that if 'Gertrude is a sheep' or 'Brian is yellow' or both then John left the milk.
We believe that either 'Julius is a lion' or 'Brian is yellow' but not both.
We believe that either 'Julius is a lion' or 'Brian is yellow' but not both.
Chances are slight that either 'Julius is a lion' or 'Brian 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
  0.1::factA.
  0.9::factB.
  0.2::factC.
  %hop1
  0.1::factX:-or(factA,factC).
  0.1::factY:-and(factA,factC).
  0.2::factZ:-or(factA,factB).

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

  query(conclusion).
  
Either 'Julius is a lion' or 'Brian is yellow' but not both.