Chances are about even that Greg is a rhino. It is highly likely that Mary put down the apple. Chances are about even that Julius is a swan. There is little chance that if 'Julius is a swan' or 'Mary put down the apple' or both then John left the milk. It is impossible that if 'Julius is a swan and Mary put down the apple' then Brian is green. There is a very good chance that if 'Julius is a swan' or 'Mary put down the apple' or both then Bernhard is a frog.
Chances are slight that either 'Bernhard is a frog' or 'Brian is green' but not both.
We believe that either 'Bernhard is a frog' or 'Brian is green' but not both.
Chances are slight that either 'Bernhard is a frog' or 'Brian 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
  0.5::factA.
  0.9::factB.
  0.5::factC.
  %hop1
  0.1::factX:-or(factC,factB).
  0.0::factY:-and(factC,factB).
  0.8::factZ:-or(factC,factB).

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

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