There is a very good chance that Yann is thirsty. It is certain that Mary went to the hallway. It is probably not the case that Julius is a swan. There is a very good chance that if 'Yann is thirsty' or 'Mary went to the hallway' or both then Lily is a rhino. Chances are slight that if 'Julius is a swan and Mary went to the hallway' then Bernhard is white. Chances are about even that if either 'Mary went to the hallway' or 'Julius is a swan' but not both then John dropped the apple.
There is almost no chance that 'Julius is a swan' or 'John dropped the apple' or both.
There is a better than even chance that 'Julius is a swan' or 'John dropped the apple' or both.
There is almost no chance that 'Julius is a swan' or 'John dropped the apple' 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
  0.8::factA.
  1.0::factB.
  0.25::factC.
  %hop1
  0.8::factX:-or(factA,factB).
  0.1::factY:-and(factC,factB).
  0.5::factZ:-xor(factB,factC).

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

  query(conclusion).
  
'Julius is a swan' or 'John dropped the apple' or both.