Chances are slight that Greg is white. It is highly likely that Sandra left the apple. We doubt that Fred dropped the milk. It is impossible that if either 'Sandra left the apple' or 'Greg is white' but not both then Mary put down the apple. It is impossible that if 'Greg is white and Fred dropped the milk' then Julius is a swan. Chances are slight that if either 'Greg is white' or 'Fred dropped the milk' but not both then Brian is a frog.
There is a very good chance that 'Fred dropped the milk and Sandra left the apple'.
We doubt that 'Fred dropped the milk and Sandra left the apple'.
There is a very good chance that 'Fred dropped the milk and Sandra left the apple'.

  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
  0.1::factA.
  0.9::factB.
  0.2::factC.
  %hop1
  0.0::factX:-xor(factB,factA).
  0.0::factY:-and(factA,factC).
  0.1::factZ:-xor(factA,factC).

  %hop2
  conclusion:-and(factC,factB).

  query(conclusion).
  
'Fred dropped the milk and Sandra left the apple'.