It is probably not the case that John went to the garden. It is highly unlikely that Emily is a cat. It is highly likely that Lily is green. It is likely that if 'Lily is green and John went to the garden' then Bernhard is a swan. We doubt that if 'Emily is a cat' or 'John went to the garden' or both then Sandra left the milk. There is a better than even chance that if either 'John went to the garden' or 'Emily is a cat' but not both then Fred is in the cinema.
It is certain that either 'Sandra left the milk' or 'Bernhard is a swan' but not both.
We doubt that either 'Sandra left the milk' or 'Bernhard is a swan' but not both.
It is certain that either 'Sandra left the milk' or 'Bernhard is a swan' 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.
  0.05::factB.
  0.9::factC.
  %hop1
  0.7::factX:-and(factC,factA).
  0.2::factY:-or(factB,factA).
  0.6::factZ:-xor(factA,factB).

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

  query(conclusion).
  
Either 'Sandra left the milk' or 'Bernhard is a swan' but not both.