Chances are about even that Sandra dropped the milk. It is highly likely that Bernhard is a rhino. It is unlikely that Greg is white. It is probably not the case that if 'Sandra dropped the milk and Greg is white' then Emily is a sheep. It is probably not the case that if 'Greg is white and Bernhard is a rhino' then John moved to the office. It is unlikely that if 'Greg is white' or 'Sandra dropped the milk' or both then Brian is a swan.
Chances are slight that either 'Bernhard is a rhino' or 'Greg is white' but not both.
We believe that either 'Bernhard is a rhino' or 'Greg is white' but not both.
Chances are slight that either 'Bernhard is a rhino' or 'Greg 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
  0.5::factA.
  0.9::factB.
  0.2::factC.
  %hop1
  0.25::factX:-and(factA,factC).
  0.25::factY:-and(factC,factB).
  0.2::factZ:-or(factC,factA).

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

  query(conclusion).
  
Either 'Bernhard is a rhino' or 'Greg is white' but not both.