Chances are about even that John went to the hallway. It is improbable that Jeff left the apple. There is almost no chance that Lily is gray. We believe that if 'Lily is gray and John went to the hallway' then Bill moved to the office. It is improbable that if either 'Lily is gray' or 'John went to the hallway' but not both then Sandra took the football. It is probably not the case that if 'John went to the hallway and Jeff left the apple' then Bernhard is a swan.
It is certain that 'Jeff left the apple' or 'Bernhard is a swan' or both.
Chances are slight that 'Jeff left the apple' or 'Bernhard is a swan' or both.
It is certain that 'Jeff left the apple' or 'Bernhard is a swan' 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.
  0.02::factC.
  %hop1
  0.75::factX:-and(factC,factA).
  0.1::factY:-xor(factC,factA).
  0.25::factZ:-and(factA,factB).

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

  query(conclusion).
  
'Jeff left the apple' or 'Bernhard is a swan' or both.