It is certain that John discarded the apple. We believe that Mary grabbed the milk. We believe that Julius is a lion. There is little chance that if 'Mary grabbed the milk and John discarded the apple' then Brian is white. There is little chance that if 'John discarded the apple' or 'Julius is a lion' or both then Sandra left the football. It is unlikely that if either 'John discarded the apple' or 'Mary grabbed the milk' but not both then Lily is a swan.
It is probably not the case that either 'Julius is a lion' or 'John discarded the apple' but not both.
It is probably not the case that either 'Julius is a lion' or 'John discarded the apple' but not both.
It is certain that either 'Julius is a lion' 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
  1.0::factA.
  0.75::factB.
  0.75::factC.
  %hop1
  0.1::factX:-and(factB,factA).
  0.1::factY:-or(factA,factC).
  0.2::factZ:-xor(factA,factB).

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

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