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

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

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