We believe that Greg is a swan. It is unlikely that Sandra took the apple. It is certain that Brian is a frog. There is a very good chance that if either 'Greg is a swan' or 'Sandra took the apple' but not both then Bernhard is green. It is highly unlikely that if 'Brian is a frog' or 'Greg is a swan' or both then Jeff discarded the milk. There is little chance that if 'Greg is a swan and Brian is a frog' then Mary dropped the milk.
It is probably not the case that either 'Bernhard is green' or 'Greg is a swan' but not both.
It is probably not the case that either 'Bernhard is green' or 'Greg is a swan' but not both.
It is probable that either 'Bernhard is green' 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
  0.75::factA.
  0.2::factB.
  1.0::factC.
  %hop1
  0.8::factX:-xor(factA,factB).
  0.05::factY:-or(factC,factA).
  0.1::factZ:-and(factA,factC).

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

  query(conclusion).
  
Either 'Bernhard is green' or 'Greg is a swan' but not both.