There is little chance that Bernhard is a swan. It is likely that Mary went to the garden. It is impossible that Brian is a frog. We doubt that if 'Mary went to the garden' or 'Brian is a frog' or both then John picked up the apple. Chances are slight that if either 'Bernhard is a swan' or 'Mary went to the garden' but not both then Julius is a lion. It is improbable that if either 'Bernhard is a swan' or 'Mary went to the garden' but not both then Lily is green.
We believe that either 'Brian is a frog' or 'Lily is green' but not both.
It is highly unlikely that either 'Brian is a frog' or 'Lily is green' but not both.
We believe that either 'Brian is a frog' or 'Lily is green' 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
  0.1::factA.
  0.7::factB.
  0.0::factC.
  %hop1
  0.2::factX:-or(factB,factC).
  0.1::factY:-xor(factA,factB).
  0.1::factZ:-xor(factA,factB).

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

  query(conclusion).
  
Either 'Brian is a frog' or 'Lily is green' but not both.