Chances are slight that Greg is a swan. We believe that Gertrude is a mouse. It is probable that Bernhard is white. It is highly unlikely that if 'Gertrude is a mouse and Greg is a swan' then Lily is green. We believe that if either 'Greg is a swan' or 'Bernhard is white' but not both then Sandra got the milk. Chances are about even that if 'Gertrude is a mouse' or 'Greg is a swan' or both then John went to the office.
Chances are about even that either 'John went to the office' or 'Greg is a swan' but not both.
Chances are about even that either 'John went to the office' or 'Greg is a swan' but not both.
It is certain that either 'John went to the office' or 'Greg 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
  0.1::factA.
  0.75::factB.
  0.7::factC.
  %hop1
  0.05::factX:-and(factB,factA).
  0.75::factY:-xor(factA,factC).
  0.5::factZ:-or(factB,factA).

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

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