context: It is likely that Bernhard is a swan. It is likely that Mary dropped the milk. It is certain that Greg is green. It is probably the case that if either 'Bernhard is a swan' or 'Greg is green' but not both then Julius is a swan. We doubt that if either 'Bernhard is a swan' or 'Greg is green' but not both then Brian is yellow. Chances are slight that if 'Bernhard is a swan and Greg is green' then Winona is a cat.
hypothesis: It is impossible that either 'Bernhard is a swan' or 'Julius is a swan' but not both.
valid_hypothesis: It is highly likely that either 'Bernhard is a swan' or 'Julius is a swan' but not both.
invalid_hypothesis: It is impossible that either 'Bernhard is a swan' or 'Julius is a swan' 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
    0.7::factA.
    0.7::factB.
    1.0::factC.
    %hop1
    0.7::factX:-xor(factA,factC).
    0.2::factY:-xor(factA,factC).
    0.1::factZ:-and(factA,factC).
  
    %hop2
    conclusion:-xor(factA,factX).
  
    query(conclusion).
    
hypothesis_assertion: Either 'Bernhard is a swan' or 'Julius is a swan' but not both.