It is highly likely that Mary took the milk. It is certain that Bernhard is a swan. We believe that Julius is gray. Chances are slight that if 'Mary took the milk' or 'Bernhard is a swan' or both then Lily is yellow. It is likely that if either 'Bernhard is a swan' or 'Mary took the milk' but not both then Greg is a frog. There is little chance that if either 'Julius is gray' or 'Bernhard is a swan' but not both then Gertrude is a wolf.
It is impossible that 'Gertrude is a wolf and Greg is a frog'.
It is impossible that 'Gertrude is a wolf and Greg is a frog'.
There is a very good chance that 'Gertrude is a wolf and Greg is a frog'.

  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
  0.9::factA.
  1.0::factB.
  0.75::factC.
  %hop1
  0.1::factX:-or(factA,factB).
  0.7::factY:-xor(factB,factA).
  0.1::factZ:-xor(factC,factB).

  %hop2
  conclusion:-and(factZ,factY).

  query(conclusion).
  
'Gertrude is a wolf and Greg is a frog'.