It is improbable that Jessica is a cat. It is certain that Mary discarded the milk. It is likely that Lily is green. It is probable that if 'Jessica is a cat and Lily is green' then Brian is white. There is almost no chance that if 'Jessica is a cat and Lily is green' then Greg is a swan. It is certain that if 'Lily is green and Mary discarded the milk' then Julius is yellow.
It is unlikely that either 'Greg is a swan' or 'Mary discarded the milk' but not both.
It is certain that either 'Greg is a swan' or 'Mary discarded the milk' but not both.
It is unlikely that either 'Greg is a swan' or 'Mary discarded 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
  0.1::factA.
  1.0::factB.
  0.7::factC.
  %hop1
  0.7::factX:-and(factA,factC).
  0.02::factY:-and(factA,factC).
  1.0::factZ:-and(factC,factB).

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

  query(conclusion).
  
Either 'Greg is a swan' or 'Mary discarded the milk' but not both.