Chances are about even that Bernhard is gray. It is improbable that Sandra left the milk. It is highly unlikely that Jeff went to the hallway. It is highly likely that if 'Sandra left the milk and Jeff went to the hallway' then Julius is a lion. We believe that if either 'Sandra left the milk' or 'Jeff went to the hallway' but not both then Gertrude is a mouse. There is almost no chance that if 'Jeff went to the hallway' or 'Sandra left the milk' or both then Brian is white.
It is highly unlikely that 'Brian is white' or 'Jeff went to the hallway' or both.
It is highly unlikely that 'Brian is white' or 'Jeff went to the hallway' or both.
It is likely that 'Brian is white' or 'Jeff went to the hallway' or both.

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

  %hop2
  conclusion:-or(factZ,factC).

  query(conclusion).
  
'Brian is white' or 'Jeff went to the hallway' or both.