It is highly likely that John dropped the apple. There is a better than even chance that Brian is a swan. It is certain that Greg is a lion. It is probable that if 'Greg is a lion and Brian is a swan' then Sandra got the football. It is probably not the case that if 'Brian is a swan' or 'John dropped the apple' or both then Mary discarded the milk. It is unlikely that if 'John dropped the apple and Greg is a lion' then Lily is a frog.
There is a very good chance that either 'Lily is a frog' or 'Greg is a lion' but not both.
There is a very good chance that either 'Lily is a frog' or 'Greg is a lion' but not both.
We doubt that either 'Lily is a frog' or 'Greg is a lion' 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
  0.9::factA.
  0.6::factB.
  1.0::factC.
  %hop1
  0.7::factX:-and(factC,factB).
  0.25::factY:-or(factB,factA).
  0.2::factZ:-and(factA,factC).

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

  query(conclusion).
  
Either 'Lily is a frog' or 'Greg is a lion' but not both.