It is highly likely that Brian is white. It is impossible that Sandra left the football. It is probably not the case that John put down the apple. It is improbable that if 'Sandra left the football and Brian is white' then Greg is a rhino. We believe that if either 'Brian is white' or 'John put down the apple' but not both then Bernhard is white. It is highly unlikely that if either 'Brian is white' or 'John put down the apple' but not both then Mary went to the hallway.
We believe that either 'Bernhard is white' or 'John put down the apple' but not both.
We believe that either 'Bernhard is white' or 'John put down the apple' but not both.
We doubt that either 'Bernhard is white' or 'John put down the apple' 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
  0.9::factA.
  0.0::factB.
  0.25::factC.
  %hop1
  0.1::factX:-and(factB,factA).
  0.75::factY:-xor(factA,factC).
  0.05::factZ:-xor(factA,factC).

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

  query(conclusion).
  
Either 'Bernhard is white' or 'John put down the apple' but not both.