Chances are about even that Greg is a frog. It is highly unlikely that Sandra left the milk. It is likely that Lily is a frog. It is certain that if 'Sandra left the milk' or 'Greg is a frog' or both then Yann is hungry. We believe that if either 'Sandra left the milk' or 'Lily is a frog' but not both then Brian is white. It is almost certain that if either 'Lily is a frog' or 'Greg is a frog' but not both then Mary grabbed the milk.
We doubt that 'Greg is a frog' or 'Mary grabbed the milk' or both.
There is a very good chance that 'Greg is a frog' or 'Mary grabbed the milk' or both.
We doubt that 'Greg is a frog' or 'Mary grabbed the milk' or 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.
  0.7::factC.
  %hop1
  1.0::factX:-or(factB,factA).
  0.75::factY:-xor(factB,factC).
  0.95::factZ:-xor(factC,factA).

  %hop2
  conclusion:-or(factA,factZ).

  query(conclusion).
  
'Greg is a frog' or 'Mary grabbed the milk' or both.