It is certain that Mary left the football. We doubt that Sandra got the milk. It is highly unlikely that Bernhard is gray. Chances are slight that if either 'Bernhard is gray' or 'Sandra got the milk' but not both then Gertrude is a cat. Chances are about even that if either 'Bernhard is gray' or 'Mary left the football' but not both then Greg is yellow. It is highly likely that if 'Bernhard is gray and Sandra got the milk' then John picked up the apple.
It is improbable that 'Mary left the football' or 'Greg is yellow' or both.
It is certain that 'Mary left the football' or 'Greg is yellow' or both.
It is improbable that 'Mary left the football' or 'Greg is yellow' 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
  1.0::factA.
  0.2::factB.
  0.05::factC.
  %hop1
  0.1::factX:-xor(factC,factB).
  0.5::factY:-xor(factC,factA).
  0.9::factZ:-and(factC,factB).

  %hop2
  conclusion:-or(factA,factY).

  query(conclusion).
  
'Mary left the football' or 'Greg is yellow' or both.