It is unlikely that Julius is a swan. There is little chance that Lily is a lion. It is probably not the case that Bernhard is a frog. It is highly unlikely that if 'Bernhard is a frog' or 'Lily is a lion' or both then John moved to the garden. Chances are about even that if 'Bernhard is a frog and Lily is a lion' then Greg is yellow. There is little chance that if 'Lily is a lion' or 'Julius is a swan' or both then Mary went to the garden.
We believe that either 'Mary went to the garden' or 'Julius is a swan' but not both.
We doubt that either 'Mary went to the garden' or 'Julius is a swan' but not both.
We believe that either 'Mary went to the garden' 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
  0.2::factA.
  0.1::factB.
  0.25::factC.
  %hop1
  0.05::factX:-or(factC,factB).
  0.5::factY:-and(factC,factB).
  0.1::factZ:-or(factB,factA).

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

  query(conclusion).
  
Either 'Mary went to the garden' or 'Julius is a swan' but not both.