It is probably not the case that John grabbed the apple. Chances are about even that Greg is yellow. It is improbable that Emily is a cat. It is highly unlikely that if 'Emily is a cat' or 'Greg is yellow' or both then Sandra took the milk. We believe that if 'John grabbed the apple' or 'Greg is yellow' or both then Winona is a mouse. It is highly unlikely that if either 'Emily is a cat' or 'Greg is yellow' but not both then Bernhard is a rhino.
It is certain that 'Emily is a cat' or 'Sandra took the milk' or both.
It is improbable that 'Emily is a cat' or 'Sandra took the milk' or both.
It is certain that 'Emily is a cat' or 'Sandra took 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
  0.25::factA.
  0.5::factB.
  0.1::factC.
  %hop1
  0.05::factX:-or(factC,factB).
  0.75::factY:-or(factA,factB).
  0.05::factZ:-xor(factC,factB).

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

  query(conclusion).
  
'Emily is a cat' or 'Sandra took the milk' or both.