We doubt that Julius is gray. It is highly likely that Brian is white. We believe that Greg is green. There is a very good chance that if 'Brian is white and Greg is green' then Sandra took the milk. There is almost no chance that if 'Greg is green' or 'Brian is white' or both then John went to the garden. We doubt that if either 'Greg is green' or 'Brian is white' but not both then Mary left the milk.
Chances are about even that 'Sandra took the milk and Brian is white'.
Chances are about even that 'Sandra took the milk and Brian is white'.
It is certain that 'Sandra took the milk and Brian is white'.

  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.
  0.9::factB.
  0.75::factC.
  %hop1
  0.8::factX:-and(factB,factC).
  0.02::factY:-or(factC,factB).
  0.2::factZ:-xor(factC,factB).

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

  query(conclusion).
  
'Sandra took the milk and Brian is white'.