There is little chance that Greg is yellow. It is almost certain that Lily is a swan. We doubt that Sandra dropped the milk. We doubt that if 'Sandra dropped the milk' or 'Greg is yellow' or both then Gertrude is a mouse. There is a very good chance that if 'Lily is a swan' or 'Greg is yellow' or both then Mary went to the bedroom. Chances are slight that if either 'Lily is a swan' or 'Greg is yellow' but not both then Brian is a frog.
It is probably not the case that either 'Sandra dropped the milk' or 'Lily is a swan' but not both.
We believe that either 'Sandra dropped the milk' or 'Lily is a swan' but not both.
It is probably not the case that either 'Sandra dropped the milk' or 'Lily 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
  0.1::factA.
  0.95::factB.
  0.2::factC.
  %hop1
  0.2::factX:-or(factC,factA).
  0.8::factY:-or(factB,factA).
  0.1::factZ:-xor(factB,factA).

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

  query(conclusion).
  
Either 'Sandra dropped the milk' or 'Lily is a swan' but not both.