We believe that Bernhard is white. We doubt that Bill went to the garden. There is a very good chance that Greg is a swan. We believe that if 'Bernhard is white and Greg is a swan' then Lily is a rhino. There is a very good chance that if either 'Bernhard is white' or 'Greg is a swan' but not both then Daniel left the apple. We believe that if 'Greg is a swan and Bernhard is white' then Mary dropped the apple.
We believe that either 'Greg is a swan' or 'Daniel left the apple' but not both.
We believe that either 'Greg is a swan' or 'Daniel left the apple' but not both.
We doubt that either 'Greg is a swan' or 'Daniel 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
  0.75::factA.
  0.2::factB.
  0.8::factC.
  %hop1
  0.75::factX:-and(factA,factC).
  0.8::factY:-xor(factA,factC).
  0.75::factZ:-and(factC,factA).

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

  query(conclusion).
  
Either 'Greg is a swan' or 'Daniel left the apple' but not both.