Chances are about even that Jeff discarded the milk. It is improbable that Fred is in the school. Chances are about even that Greg is a swan. It is improbable that if 'Greg is a swan' or 'Jeff discarded the milk' or both then Brian is white. We believe that if 'Fred is in the school and Jeff discarded the milk' then Bernhard is yellow. It is probably not the case that if 'Fred is in the school and Greg is a swan' then John dropped the apple.
It is highly unlikely that 'Brian is white and Jeff discarded the milk'.
It is highly unlikely that 'Brian is white and Jeff discarded the milk'.
It is almost certain that 'Brian is white and Jeff discarded the milk'.

  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
  0.5::factA.
  0.1::factB.
  0.5::factC.
  %hop1
  0.1::factX:-or(factC,factA).
  0.75::factY:-and(factB,factA).
  0.25::factZ:-and(factB,factC).

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

  query(conclusion).
  
'Brian is white and Jeff discarded the milk'.