It is unlikely that Greg is a swan. There is little chance that Julius is white. Chances are about even that Brian is yellow. It is probable that if 'Julius is white' or 'Greg is a swan' or both then Winona is a mouse. It is impossible that if 'Julius is white and Greg is a swan' then Jason is tired. There is a very good chance that if either 'Greg is a swan' or 'Julius is white' but not both then John dropped the apple.
Chances are about even that either 'Greg is a swan' or 'Brian is yellow' but not both.
Chances are about even that either 'Greg is a swan' or 'Brian is yellow' but not both.
There is almost no chance that either 'Greg is a swan' or 'Brian is yellow' 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
  0.2::factA.
  0.1::factB.
  0.5::factC.
  %hop1
  0.7::factX:-or(factB,factA).
  0.0::factY:-and(factB,factA).
  0.8::factZ:-xor(factA,factB).

  %hop2
  conclusion:-xor(factA,factC).

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