There is little chance that Julius is gray. It is highly likely that Greg is a swan. It is probably not the case that Daniel left the football. Chances are about even that if 'Julius is gray and Greg is a swan' then Bernhard is gray. There is a better than even chance that if 'Julius is gray' or 'Daniel left the football' or both then Brian is a rhino. Chances are about even that if 'Daniel left the football' or 'Greg is a swan' or both then John went to the hallway.
It is highly unlikely that either 'Greg is a swan' or 'Brian is a rhino' but not both.
We believe that either 'Greg is a swan' or 'Brian is a rhino' but not both.
It is highly unlikely that either 'Greg is a swan' or 'Brian 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
  0.1::factA.
  0.9::factB.
  0.25::factC.
  %hop1
  0.5::factX:-and(factA,factB).
  0.6::factY:-or(factA,factC).
  0.5::factZ:-or(factC,factB).

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

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