It is certain that Julius is gray. Chances are about even that Greg is a swan. It is highly likely that John went to the hallway. It is impossible that if 'John went to the hallway and Greg is a swan' then Lily is a rhino. We doubt that if 'Julius is gray and Greg is a swan' then Sandra took the milk. It is highly likely that if 'Julius is gray' or 'John went to the hallway' or both then Brian is a lion.
Chances are about even that either 'Greg is a swan' or 'Lily is a rhino' but not both.
Chances are about even that either 'Greg is a swan' or 'Lily is a rhino' but not both.
It is almost certain that either 'Greg is a swan' or 'Lily is a rhino' but not 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.
  0.5::factB.
  0.9::factC.
  %hop1
  0.0::factX:-and(factC,factB).
  0.2::factY:-and(factA,factB).
  0.9::factZ:-or(factA,factC).

  %hop2
  conclusion:-xor(factB,factX).

  query(conclusion).
  
Either 'Greg is a swan' or 'Lily is a rhino' but not both.