Chances are about even that Greg is white. It is highly unlikely that John grabbed the apple. There is a very good chance that Brian is a frog. It is probable that if 'John grabbed the apple' or 'Brian is a frog' or both then Mary went to the garden. Chances are slight that if 'Brian is a frog and John grabbed the apple' then Lily is a swan. It is almost certain that if either 'Brian is a frog' or 'John grabbed the apple' but not both then Jessica is a mouse.
We doubt that either 'Brian is a frog' or 'Lily is a swan' but not both.
There is a very good chance that either 'Brian is a frog' or 'Lily is a swan' but not both.
We doubt that either 'Brian is a frog' or 'Lily is a swan' 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.
  0.05::factB.
  0.8::factC.
  %hop1
  0.7::factX:-or(factB,factC).
  0.1::factY:-and(factC,factB).
  0.95::factZ:-xor(factC,factB).

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

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