It is probably not the case that John discarded the apple. It is highly unlikely that Lily is a frog. Chances are slight that Brian is a lion. There is little chance that if 'Lily is a frog' or 'John discarded the apple' or both then Bernhard is a swan. It is probably not the case that if 'Brian is a lion and John discarded the apple' then Sandra dropped the milk. It is unlikely that if 'John discarded the apple' or 'Lily is a frog' or both then Mary went to the office.
It is almost certain that either 'Bernhard is a swan' or 'John discarded the apple' but not both.
It is probably not the case that either 'Bernhard is a swan' or 'John discarded the apple' but not both.
It is almost certain that either 'Bernhard is a swan' or 'John discarded the apple' 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.
  0.1::factC.
  %hop1
  0.1::factX:-or(factB,factA).
  0.25::factY:-and(factC,factA).
  0.2::factZ:-or(factA,factB).

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

  query(conclusion).
  
Either 'Bernhard is a swan' or 'John discarded the apple' but not both.