It is highly likely that Greg is a swan. It is improbable that Gertrude is a mouse. It is highly likely that Brian is a rhino. There is almost no chance that if 'Greg is a swan and Gertrude is a mouse' then Lily is yellow. Chances are slight that if 'Greg is a swan and Gertrude is a mouse' then Bernhard is gray. Chances are slight that if either 'Greg is a swan' or 'Gertrude is a mouse' but not both then Mary dropped the milk.
We doubt that either 'Brian is a rhino' or 'Greg is a swan' but not both.
We doubt that either 'Brian is a rhino' or 'Greg is a swan' but not both.
We believe that either 'Brian is a rhino' 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
  0.9::factA.
  0.1::factB.
  0.9::factC.
  %hop1
  0.02::factX:-and(factA,factB).
  0.1::factY:-and(factA,factB).
  0.1::factZ:-xor(factA,factB).

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

  query(conclusion).
  
Either 'Brian is a rhino' or 'Greg is a swan' but not both.