It is impossible that Julius is a swan. It is highly likely that Brian is green. Chances are about even that Fred dropped the apple. There is a better than even chance that if either 'Brian is green' or 'Fred dropped the apple' but not both then Greg is a frog. It is probable that if 'Brian is green' or 'Julius is a swan' or both then Mary discarded the milk. There is almost no chance that if either 'Brian is green' or 'Fred dropped the apple' but not both then John went to the office.
It is highly likely that either 'Brian is green' or 'Julius is a swan' but not both.
It is highly likely that either 'Brian is green' or 'Julius is a swan' but not both.
There is little chance that either 'Brian is green' or 'Julius is a swan' 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
  0.0::factA.
  0.9::factB.
  0.5::factC.
  %hop1
  0.6::factX:-xor(factB,factC).
  0.7::factY:-or(factB,factA).
  0.02::factZ:-xor(factB,factC).

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

  query(conclusion).
  
Either 'Brian is green' or 'Julius is a swan' but not both.