It is likely that Mary left the milk. There is a very good chance that John went to the kitchen. There is a very good chance that Fred put down the apple. It is improbable that if 'Fred put down the apple' or 'Mary left the milk' or both then Julius is a frog. It is impossible that if 'Mary left the milk and Fred put down the apple' then Bernhard is a swan. We believe that if either 'John went to the kitchen' or 'Fred put down the apple' but not both then Greg is a rhino.
It is improbable that either 'Julius is a frog' or 'John went to the kitchen' but not both.
We believe that either 'Julius is a frog' or 'John went to the kitchen' but not both.
It is improbable that either 'Julius is a frog' or 'John went to the kitchen' 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.
  0.8::factB.
  0.8::factC.
  %hop1
  0.1::factX:-or(factC,factA).
  0.0::factY:-and(factA,factC).
  0.75::factZ:-xor(factB,factC).

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

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