It is probably not the case that Greg is a swan. It is almost certain that Sandra got the football. It is impossible that Emily is a sheep. There is a very good chance that if 'Sandra got the football' or 'Greg is a swan' or both then Brian is yellow. We doubt that if 'Greg is a swan and Sandra got the football' then Bernhard is gray. It is certain that if 'Sandra got the football' or 'Greg is a swan' or both then Fred discarded the apple.
It is likely that either 'Sandra got the football' or 'Greg is a swan' but not both.
It is likely that either 'Sandra got the football' or 'Greg is a swan' but not both.
There is almost no chance that either 'Sandra got the football' 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
  0.25::factA.
  0.95::factB.
  0.0::factC.
  %hop1
  0.8::factX:-or(factB,factA).
  0.2::factY:-and(factA,factB).
  1.0::factZ:-or(factB,factA).

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

  query(conclusion).
  
Either 'Sandra got the football' or 'Greg is a swan' but not both.