It is improbable that Julius is a swan. Chances are slight that Fred left the apple. It is unlikely that Greg is a rhino. It is certain that if 'Fred left the apple' or 'Julius is a swan' or both then Gertrude is a wolf. It is likely that if 'Greg is a rhino' or 'Julius is a swan' or both then Bernhard is a frog. It is unlikely that if 'Greg is a rhino and Julius is a swan' then John moved to the garden.
It is probably not the case that either 'Bernhard is a frog' or 'Fred left the apple' but not both.
It is probably not the case that either 'Bernhard is a frog' or 'Fred left the apple' but not both.
We believe that either 'Bernhard is a frog' or 'Fred left the apple' 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.
  0.1::factB.
  0.2::factC.
  %hop1
  1.0::factX:-or(factB,factA).
  0.7::factY:-or(factC,factA).
  0.2::factZ:-and(factC,factA).

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

  query(conclusion).
  
Either 'Bernhard is a frog' or 'Fred left the apple' but not both.