It is highly unlikely that John went to the bedroom. It is impossible that Greg is a frog. There is almost no chance that Bernhard is green. It is probably not the case that if either 'John went to the bedroom' or 'Greg is a frog' but not both then Julius is a swan. Chances are about even that if 'Bernhard is green' or 'Greg is a frog' or both then Fred left the football. Chances are about even that if 'John went to the bedroom and Greg is a frog' then Sandra dropped the apple.
There is almost no chance that either 'Greg is a frog' or 'Julius is a swan' but not both.
There is almost no chance that either 'Greg is a frog' or 'Julius is a swan' but not both.
It is certain that either 'Greg is a frog' or 'Julius is a swan' 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
  0.05::factA.
  0.0::factB.
  0.02::factC.
  %hop1
  0.25::factX:-xor(factA,factB).
  0.5::factY:-or(factC,factB).
  0.5::factZ:-and(factA,factB).

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

  query(conclusion).
  
Either 'Greg is a frog' or 'Julius is a swan' but not both.