It is highly likely that Mary grabbed the milk. There is a better than even chance that Sandra left the milk. We believe that Bernhard is a frog. We believe that if 'Mary grabbed the milk and Bernhard is a frog' then Lily is a swan. There is little chance that if 'Sandra left the milk and Bernhard is a frog' then Daniel got the football. It is likely that if 'Sandra left the milk' or 'Bernhard is a frog' or both then Jessica is a cat.
Chances are about even that either 'Sandra left the milk' or 'Lily is a swan' but not both.
Chances are about even that either 'Sandra left the milk' or 'Lily is a swan' but not both.
There is almost no chance that either 'Sandra left the milk' or 'Lily is a swan' 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
  0.9::factA.
  0.6::factB.
  0.75::factC.
  %hop1
  0.75::factX:-and(factA,factC).
  0.1::factY:-and(factB,factC).
  0.7::factZ:-or(factB,factC).

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

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