It is likely that Bill went to the garden. It is probably the case that Greg is a swan. It is improbable that Mary got the milk. We believe that if 'Greg is a swan and Mary got the milk' then John left the football. It is almost certain that if 'Mary got the milk' or 'Bill went to the garden' or both then Brian is a frog. It is unlikely that if 'Mary got the milk' or 'Bill went to the garden' or both then Bernhard is green.
Chances are about even that either 'Greg is a swan' or 'Bill went to the garden' but not both.
Chances are about even that either 'Greg is a swan' or 'Bill went to the garden' but not both.
There is almost no chance that either 'Greg is a swan' or 'Bill 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.
  0.1::factC.
  %hop1
  0.75::factX:-and(factB,factC).
  0.95::factY:-or(factC,factA).
  0.2::factZ:-or(factC,factA).

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

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