It is likely that Greg is a frog. We believe that Brian is green. It is likely that Julius is a swan. We doubt that if 'Greg is a frog' or 'Brian is green' or both then John picked up the apple. We believe that if 'Brian is green and Julius is a swan' then Gertrude is a sheep. It is highly likely that if either 'Julius is a swan' or 'Brian is green' but not both then Mary dropped the milk.
There is a better than even chance that either 'Greg is a frog' or 'John picked up the apple' but not both.
There is a better than even chance that either 'Greg is a frog' or 'John picked up the apple' but not both.
It is improbable that either 'Greg is a frog' or 'John picked up the apple' 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.
  0.75::factB.
  0.7::factC.
  %hop1
  0.2::factX:-or(factA,factB).
  0.75::factY:-and(factB,factC).
  0.9::factZ:-xor(factC,factB).

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

  query(conclusion).
  
Either 'Greg is a frog' or 'John picked up the apple' but not both.