Chances are about even that Sandra dropped the milk. It is highly likely that Greg is a swan. Chances are about even that John moved to the office. It is probably not the case that if 'Greg is a swan and John moved to the office' then Bernhard is a swan. It is impossible that if 'Sandra dropped the milk and John moved to the office' then Lily is gray. There is a better than even chance that if 'Greg is a swan and Sandra dropped the milk' then Brian is a frog.
It is improbable that either 'Greg is a swan' or 'Brian is a frog' but not both.
There is a better than even chance that either 'Greg is a swan' or 'Brian is a frog' but not both.
It is improbable that either 'Greg is a swan' or 'Brian is a frog' 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
  0.25::factX:-and(factB,factC).
  0.0::factY:-and(factA,factC).
  0.6::factZ:-and(factB,factA).

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

  query(conclusion).
  
Either 'Greg is a swan' or 'Brian is a frog' but not both.