It is probably the case that Bernhard is yellow. We doubt that Mary got the football. There is a better than even chance that Jeff left the apple. It is impossible that if 'Bernhard is yellow and Jeff left the apple' then Fred dropped the milk. There is almost no chance that if either 'Mary got the football' or 'Jeff left the apple' but not both then Lily is a swan. It is almost certain that if either 'Mary got the football' or 'Jeff left the apple' but not both then Brian is gray.
It is highly unlikely that 'Fred dropped the milk' or 'Brian is gray' or both.
Chances are about even that 'Fred dropped the milk' or 'Brian is gray' or both.
It is highly unlikely that 'Fred dropped the milk' or 'Brian is gray' 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
  0.7::factA.
  0.2::factB.
  0.6::factC.
  %hop1
  0.0::factX:-and(factA,factC).
  0.02::factY:-xor(factB,factC).
  0.95::factZ:-xor(factB,factC).

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

  query(conclusion).
  
'Fred dropped the milk' or 'Brian is gray' or both.