context: We doubt that Brian is a swan. It is likely that Sandra left the milk. There is a better than even chance that Mary took the milk. It is highly unlikely that if 'Sandra left the milk and Brian is a swan' then Lily is white. Chances are about even that if either 'Brian is a swan' or 'Sandra left the milk' but not both then Julius is a frog. It is almost certain that if 'Brian is a swan' or 'Sandra left the milk' or both then Bernhard is a lion.
hypothesis: Chances are about even that either 'Sandra left the milk' or 'Julius is a frog' but not both.
valid_hypothesis: Chances are about even that either 'Sandra left the milk' or 'Julius is a frog' but not both.
invalid_hypothesis: It is certain that either 'Sandra left the milk' or 'Julius is a frog' but not both.
problog: 
    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
    0.2::factA.
    0.7::factB.
    0.6::factC.
    %hop1
    0.05::factX:-and(factB,factA).
    0.5::factY:-xor(factA,factB).
    0.95::factZ:-or(factA,factB).
  
    %hop2
    conclusion:-xor(factB,factY).
  
    query(conclusion).
    
hypothesis_assertion: Either 'Sandra left the milk' or 'Julius is a frog' but not both.