Chances are slight that Gertrude is a mouse. It is highly likely that Brian is white. It is unlikely that Lily is a swan. Chances are about even that if 'Lily is a swan' or 'Brian is white' or both then Greg is a swan. There is a very good chance that if 'Gertrude is a mouse and Brian is white' then Mary went to the hallway. It is improbable that if 'Gertrude is a mouse and Lily is a swan' then John moved to the garden.
Chances are about even that 'Gertrude is a mouse' or 'Greg is a swan' or both.
Chances are about even that 'Gertrude is a mouse' or 'Greg is a swan' or both.
It is highly unlikely that 'Gertrude is a mouse' or 'Greg 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
  0.1::factA.
  0.9::factB.
  0.2::factC.
  %hop1
  0.5::factX:-or(factC,factB).
  0.8::factY:-and(factA,factB).
  0.1::factZ:-and(factA,factC).

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

  query(conclusion).
  
'Gertrude is a mouse' or 'Greg is a swan' or both.