It is probably not the case that Brian is green. There is little chance that Greg is gray. It is probably not the case that Sandra took the milk. It is likely that if 'Brian is green and Greg is gray' then John went to the garden. It is impossible that if 'Greg is gray and Brian is green' then Gertrude is a sheep. We believe that if 'Sandra took the milk' or 'Greg is gray' or both then Emily is a wolf.
There is almost no chance that either 'Brian is green' or 'Sandra took the milk' but not both.
Chances are about even that either 'Brian is green' or 'Sandra took the milk' but not both.
There is almost no chance that either 'Brian is green' or 'Sandra took the milk' 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
  0.25::factA.
  0.1::factB.
  0.25::factC.
  %hop1
  0.7::factX:-and(factA,factB).
  0.0::factY:-and(factB,factA).
  0.75::factZ:-or(factC,factB).

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

  query(conclusion).
  
Either 'Brian is green' or 'Sandra took the milk' but not both.