Chances are about even that Bernhard is a swan. It is unlikely that Jeff put down the milk. It is impossible that Julius is a frog. It is impossible that if 'Jeff put down the milk' or 'Bernhard is a swan' or both then Bill went to the office. It is likely that if 'Jeff put down the milk and Bernhard is a swan' then Lily is white. It is impossible that if 'Jeff put down the milk and Bernhard is a swan' then John dropped the milk.
It is certain that either 'Bernhard is a swan' or 'Julius is a frog' but not both.
Chances are about even that either 'Bernhard is a swan' or 'Julius is a frog' but not both.
It is certain that either 'Bernhard is a swan' or 'Julius is a frog' 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
  0.5::factA.
  0.2::factB.
  0.0::factC.
  %hop1
  0.0::factX:-or(factB,factA).
  0.7::factY:-and(factB,factA).
  0.0::factZ:-and(factB,factA).

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

  query(conclusion).
  
Either 'Bernhard is a swan' or 'Julius is a frog' but not both.