We believe that Julius is a lion. It is probably the case that John put down the apple. There is little chance that Mary dropped the milk. Chances are about even that if 'John put down the apple and Julius is a lion' then Gertrude is a cat. We believe that if 'Mary dropped the milk' or 'Julius is a lion' or both then Bill left the milk. Chances are about even that if 'Julius is a lion and Mary dropped the milk' then Lily is a rhino.
Chances are about even that either 'Julius is a lion' or 'John put down the apple' but not both.
Chances are about even that either 'Julius is a lion' or 'John put down the apple' but not both.
It is impossible that either 'Julius is a lion' or 'John put down 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
  0.75::factA.
  0.7::factB.
  0.1::factC.
  %hop1
  0.5::factX:-and(factB,factA).
  0.75::factY:-or(factC,factA).
  0.5::factZ:-and(factA,factC).

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

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