We doubt that Mary dropped the apple. There is little chance that Julius is yellow. There is a very good chance that Emily is a sheep. It is almost certain that if 'Mary dropped the apple' or 'Julius is yellow' or both then Lily is a frog. There is almost no chance that if either 'Mary dropped the apple' or 'Emily is a sheep' but not both then John moved to the office. There is almost no chance that if 'Mary dropped the apple' or 'Emily is a sheep' or both then Bernhard is a swan.
We doubt that either 'Bernhard is a swan' or 'Mary dropped the apple' but not both.
We doubt that either 'Bernhard is a swan' or 'Mary dropped the apple' but not both.
It is certain that either 'Bernhard is a swan' or 'Mary dropped 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
  0.2::factA.
  0.1::factB.
  0.8::factC.
  %hop1
  0.95::factX:-or(factA,factB).
  0.02::factY:-xor(factA,factC).
  0.02::factZ:-or(factA,factC).

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

  query(conclusion).
  
Either 'Bernhard is a swan' or 'Mary dropped the apple' but not both.