It is unlikely that Brian is a swan. There is a very good chance that Julius is yellow. It is probable that John discarded the milk. It is highly likely that if 'Brian is a swan and Julius is yellow' then Mary got the apple. We believe that if either 'Brian is a swan' or 'John discarded the milk' but not both then Greg is green. It is impossible that if either 'Brian is a swan' or 'Julius is yellow' but not both then Fred is in the office.
It is unlikely that either 'Brian is a swan' or 'Fred is in the office' but not both.
It is unlikely that either 'Brian is a swan' or 'Fred is in the office' but not both.
It is likely that either 'Brian is a swan' or 'Fred is in the office' 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.
  0.8::factB.
  0.7::factC.
  %hop1
  0.9::factX:-and(factA,factB).
  0.75::factY:-xor(factA,factC).
  0.0::factZ:-xor(factA,factB).

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

  query(conclusion).
  
Either 'Brian is a swan' or 'Fred is in the office' but not both.