There is little chance that Julius is a lion. There is almost no chance that Jessica is a mouse. It is impossible that Lily is a swan. It is improbable that if 'Jessica is a mouse' or 'Lily is a swan' or both then Greg is a swan. It is probable that if 'Jessica is a mouse and Lily is a swan' then John moved to the office. We believe that if either 'Lily is a swan' or 'Jessica is a mouse' but not both then Mary dropped the milk.
It is impossible that either 'Mary dropped the milk' or 'Jessica is a mouse' but not both.
It is impossible that either 'Mary dropped the milk' or 'Jessica is a mouse' but not both.
There is a better than even chance that either 'Mary dropped the milk' or 'Jessica is a mouse' 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.
  0.02::factB.
  0.0::factC.
  %hop1
  0.1::factX:-or(factB,factC).
  0.7::factY:-and(factB,factC).
  0.75::factZ:-xor(factC,factB).

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

  query(conclusion).
  
Either 'Mary dropped the milk' or 'Jessica is a mouse' but not both.