There is a very good chance that Greg is green. It is certain that Brian is white. We believe that Jeff discarded the milk. It is probable that if 'Brian is white' or 'Greg is green' or both then John went to the office. Chances are about even that if 'Greg is green and Brian is white' then Julius is gray. We doubt that if either 'Brian is white' or 'Jeff discarded the milk' but not both then Yann is thirsty.
There is a better than even chance that either 'Julius is gray' or 'Brian is white' but not both.
There is a better than even chance that either 'Julius is gray' or 'Brian is white' but not both.
There is almost no chance that either 'Julius is gray' or 'Brian is white' 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
  0.8::factA.
  1.0::factB.
  0.75::factC.
  %hop1
  0.7::factX:-or(factB,factA).
  0.5::factY:-and(factA,factB).
  0.2::factZ:-xor(factB,factC).

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

  query(conclusion).
  
Either 'Julius is gray' or 'Brian is white' but not both.