It is improbable that John discarded the apple. It is highly likely that Mary dropped the apple. It is likely that Greg is a frog. We doubt that if 'Greg is a frog' or 'John discarded the apple' or both then Julius is yellow. It is improbable that if either 'Mary dropped the apple' or 'Greg is a frog' but not both then Gertrude is a cat. We believe that if 'Greg is a frog' or 'Mary dropped the apple' or both then Bernhard is white.
There is little chance that 'Gertrude is a cat' or 'Bernhard is white' or both.
We believe that 'Gertrude is a cat' or 'Bernhard is white' or both.
There is little chance that 'Gertrude is a cat' or 'Bernhard is white' 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
  0.1::factA.
  0.9::factB.
  0.7::factC.
  %hop1
  0.2::factX:-or(factC,factA).
  0.1::factY:-xor(factB,factC).
  0.75::factZ:-or(factC,factB).

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

  query(conclusion).
  
'Gertrude is a cat' or 'Bernhard is white' or both.