There is a very good chance that Bernhard is a frog. It is certain that Julius is a swan. We doubt that Mary grabbed the apple. There is little chance that if 'Julius is a swan' or 'Mary grabbed the apple' or both then Gertrude is a mouse. There is little chance that if 'Julius is a swan and Mary grabbed the apple' then Lily is white. We believe that if 'Mary grabbed the apple' or 'Julius is a swan' or both then John went to the hallway.
It is highly unlikely that 'Gertrude is a mouse' or 'John went to the hallway' or both.
We believe that 'Gertrude is a mouse' or 'John went to the hallway' or both.
It is highly unlikely that 'Gertrude is a mouse' or 'John went to the hallway' or 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
  0.8::factA.
  1.0::factB.
  0.2::factC.
  %hop1
  0.1::factX:-or(factB,factC).
  0.1::factY:-and(factB,factC).
  0.75::factZ:-or(factC,factB).

  %hop2
  conclusion:-or(factX,factZ).

  query(conclusion).
  
'Gertrude is a mouse' or 'John went to the hallway' or both.