There is almost no chance that Lily is a swan. Chances are slight that Jessica is a sheep. There is a very good chance that Brian is white. It is highly unlikely that if 'Lily is a swan and Brian is white' then Julius is green. It is impossible that if 'Jessica is a sheep' or 'Lily is a swan' or both then Fred dropped the milk. It is almost certain that if 'Jessica is a sheep' or 'Brian is white' or both then Mary moved to the office.
There is a very good chance that either 'Lily is a swan' or 'Julius is green' but not both.
There is almost no chance that either 'Lily is a swan' or 'Julius is green' but not both.
There is a very good chance that either 'Lily is a swan' or 'Julius is green' 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
  0.02::factA.
  0.1::factB.
  0.8::factC.
  %hop1
  0.05::factX:-and(factA,factC).
  0.0::factY:-or(factB,factA).
  0.95::factZ:-or(factB,factC).

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

  query(conclusion).
  
Either 'Lily is a swan' or 'Julius is green' but not both.