It is likely that Bernhard is gray. Chances are slight that Gertrude is a cat. It is highly unlikely that John dropped the apple. It is certain that if either 'Gertrude is a cat' or 'John dropped the apple' but not both then Lily is yellow. We doubt that if 'John dropped the apple' or 'Gertrude is a cat' or both then Brian is a rhino. We believe that if 'John dropped the apple and Bernhard is gray' then Mary moved to the garden.
It is highly unlikely that either 'Brian is a rhino' or 'Mary moved to the garden' but not both.
It is highly unlikely that either 'Brian is a rhino' or 'Mary moved to the garden' but not both.
We believe that either 'Brian is a rhino' or 'Mary moved to the garden' 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.
  0.1::factB.
  0.05::factC.
  %hop1
  1.0::factX:-xor(factB,factC).
  0.2::factY:-or(factC,factB).
  0.75::factZ:-and(factC,factA).

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

  query(conclusion).
  
Either 'Brian is a rhino' or 'Mary moved to the garden' but not both.