Chances are about even that Mary grabbed the milk. It is probably not the case that Julius is white. It is improbable that Brian is green. There is a very good chance that if 'Brian is green' or 'Julius is white' or both then Greg is a swan. There is almost no chance that if 'Mary grabbed the milk and Brian is green' then Lily is a lion. It is improbable that if 'Julius is white' or 'Mary grabbed the milk' or both then John left the apple.
It is almost certain that either 'Greg is a swan' or 'Julius is white' but not both.
It is improbable that either 'Greg is a swan' or 'Julius is white' but not both.
It is almost certain that either 'Greg is a swan' or 'Julius is white' 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
  0.5::factA.
  0.25::factB.
  0.1::factC.
  %hop1
  0.8::factX:-or(factC,factB).
  0.02::factY:-and(factA,factC).
  0.1::factZ:-or(factB,factA).

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

  query(conclusion).
  
Either 'Greg is a swan' or 'Julius is white' but not both.