It is certain that Greg is white. We doubt that John dropped the apple. It is probably not the case that Mary grabbed the milk. Chances are about even that if 'Mary grabbed the milk and John dropped the apple' then Julius is gray. It is improbable that if 'John dropped the apple and Greg is white' then Winona is a wolf. We doubt that if either 'Greg is white' or 'John dropped the apple' but not both then Bernhard is a frog.
It is probably not the case that 'Mary grabbed the milk and Greg is white'.
It is probably not the case that 'Mary grabbed the milk and Greg is white'.
It is probably the case that 'Mary grabbed the milk and Greg 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
  1.0::factA.
  0.2::factB.
  0.25::factC.
  %hop1
  0.5::factX:-and(factC,factB).
  0.1::factY:-and(factB,factA).
  0.2::factZ:-xor(factA,factB).

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

  query(conclusion).
  
'Mary grabbed the milk and Greg is white'.