We doubt that Julius is a swan. It is highly unlikely that Greg is a lion. It is certain that Mary dropped the milk. There is almost no chance that if 'Greg is a lion and Julius is a swan' then Gertrude is a mouse. There is almost no chance that if 'Julius is a swan and Greg is a lion' then Winona is a wolf. Chances are slight that if either 'Mary dropped the milk' or 'Julius is a swan' but not both then Bernhard is a frog.
It is impossible that 'Gertrude is a mouse and Julius is a swan'.
It is impossible that 'Gertrude is a mouse and Julius is a swan'.
It is almost certain that 'Gertrude is a mouse and Julius is a swan'.

  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.
  0.05::factB.
  1.0::factC.
  %hop1
  0.02::factX:-and(factB,factA).
  0.02::factY:-and(factA,factB).
  0.1::factZ:-xor(factC,factA).

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

  query(conclusion).
  
'Gertrude is a mouse and Julius is a swan'.