It is certain that Brian is a swan. There is little chance that Greg is a rhino. It is certain that Julius is a frog. There is almost no chance that if either 'Brian is a swan' or 'Julius is a frog' but not both then Jessica is a cat. It is almost certain that if either 'Brian is a swan' or 'Julius is a frog' but not both then Sandra got the milk. There is almost no chance that if 'Brian is a swan' or 'Julius is a frog' or both then Mary went to the office.
It is certain that either 'Brian is a swan' or 'Sandra got the milk' but not both.
It is certain that either 'Brian is a swan' or 'Sandra got the milk' but not both.
Chances are slight that either 'Brian is a swan' or 'Sandra got the milk' 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.
  0.1::factB.
  1.0::factC.
  %hop1
  0.02::factX:-xor(factA,factC).
  0.95::factY:-xor(factA,factC).
  0.02::factZ:-or(factA,factC).

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

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