There is a very good chance that Greg is gray. It is likely that Jeff left the apple. It is probably the case that Mary grabbed the milk. Chances are slight that if 'Jeff left the apple' or 'Mary grabbed the milk' or both then John took the football. There is a very good chance that if either 'Mary grabbed the milk' or 'Jeff left the apple' but not both then Bernhard is a lion. Chances are about even that if 'Mary grabbed the milk' or 'Jeff left the apple' or both then Brian is yellow.
It is unlikely that 'Greg is gray' or 'Jeff left the apple' or both.
It is almost certain that 'Greg is gray' or 'Jeff left the apple' or both.
It is unlikely that 'Greg is gray' or 'Jeff left the apple' 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
  0.8::factA.
  0.7::factB.
  0.7::factC.
  %hop1
  0.1::factX:-or(factB,factC).
  0.8::factY:-xor(factC,factB).
  0.5::factZ:-or(factC,factB).

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

  query(conclusion).
  
'Greg is gray' or 'Jeff left the apple' or both.