It is likely that Greg is a swan. Chances are about even that Mary grabbed the milk. There is almost no chance that Bernhard is a frog. It is unlikely that if 'Bernhard is a frog and Mary grabbed the milk' then John went to the office. It is certain that if 'Mary grabbed the milk' or 'Bernhard is a frog' or both then Jessica is a cat. It is impossible that if either 'Mary grabbed the milk' or 'Bernhard is a frog' but not both then Lily is a lion.
Chances are about even that either 'John went to the office' or 'Jessica is a cat' but not both.
Chances are about even that either 'John went to the office' or 'Jessica is a cat' but not both.
It is certain that either 'John went to the office' or 'Jessica is a cat' 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
  0.7::factA.
  0.5::factB.
  0.02::factC.
  %hop1
  0.2::factX:-and(factC,factB).
  1.0::factY:-or(factB,factC).
  0.0::factZ:-xor(factB,factC).

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

  query(conclusion).
  
Either 'John went to the office' or 'Jessica is a cat' but not both.