We doubt that Jessica is a mouse. It is probable that Greg is a swan. There is little chance that Mary went to the garden. It is certain that if 'Jessica is a mouse and Greg is a swan' then John grabbed the apple. It is likely that if 'Mary went to the garden' or 'Greg is a swan' or both then Sandra got the milk. Chances are about even that if 'Mary went to the garden and Greg is a swan' then Brian is a lion.
We believe that either 'Brian is a lion' or 'Jessica is a mouse' but not both.
We doubt that either 'Brian is a lion' or 'Jessica is a mouse' but not both.
We believe that either 'Brian is a lion' or 'Jessica is a mouse' 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.
  0.7::factB.
  0.1::factC.
  %hop1
  1.0::factX:-and(factA,factB).
  0.7::factY:-or(factC,factB).
  0.5::factZ:-and(factC,factB).

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

  query(conclusion).
  
Either 'Brian is a lion' or 'Jessica is a mouse' but not both.