It is unlikely that Fred went to the garden. There is a very good chance that Mary dropped the milk. It is impossible that Greg is yellow. It is impossible that if 'Mary dropped the milk' or 'Greg is yellow' or both then Bill left the milk. There is a very good chance that if 'Greg is yellow' or 'Fred went to the garden' or both then John discarded the apple. It is highly unlikely that if 'Greg is yellow' or 'Mary dropped the milk' or both then Bernhard is a rhino.
We believe that 'Bill left the milk' or 'Fred went to the garden' or both.
We doubt that 'Bill left the milk' or 'Fred went to the garden' or both.
We believe that 'Bill left the milk' or 'Fred went to the garden' 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
  0.2::factA.
  0.8::factB.
  0.0::factC.
  %hop1
  0.0::factX:-or(factB,factC).
  0.8::factY:-or(factC,factA).
  0.05::factZ:-or(factC,factB).

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

  query(conclusion).
  
'Bill left the milk' or 'Fred went to the garden' or both.