It is unlikely that Julius is a swan. It is highly likely that John left the milk. It is highly unlikely that Brian is a swan. We believe that if 'Brian is a swan' or 'Julius is a swan' or both then Gertrude is a sheep. It is probably the case that if either 'Julius is a swan' or 'Brian is a swan' but not both then Lily is a lion. There is almost no chance that if 'Julius is a swan and Brian is a swan' then Mary went to the hallway.
We believe that either 'Lily is a lion' or 'John left the milk' but not both.
We believe that either 'Lily is a lion' or 'John left the milk' but not both.
Chances are slight that either 'Lily is a lion' or 'John left the milk' 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.
  0.9::factB.
  0.05::factC.
  %hop1
  0.75::factX:-or(factC,factA).
  0.7::factY:-xor(factA,factC).
  0.02::factZ:-and(factA,factC).

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

  query(conclusion).
  
Either 'Lily is a lion' or 'John left the milk' but not both.