It is probable that John got the milk. Chances are slight that Brian is yellow. Chances are about even that Greg is white. There is a very good chance that if 'Greg is white' or 'Brian is yellow' or both then Mary dropped the milk. It is almost certain that if either 'Brian is yellow' or 'John got the milk' but not both then Winona is a wolf. It is probably not the case that if either 'John got the milk' or 'Greg is white' but not both then Bernhard is a lion.
It is highly unlikely that 'Greg is white and Brian is yellow'.
It is highly unlikely that 'Greg is white and Brian is yellow'.
There is a very good chance that 'Greg is white and Brian is yellow'.

  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.
  0.1::factB.
  0.5::factC.
  %hop1
  0.8::factX:-or(factC,factB).
  0.95::factY:-xor(factB,factA).
  0.25::factZ:-xor(factA,factC).

  %hop2
  conclusion:-and(factC,factB).

  query(conclusion).
  
'Greg is white and Brian is yellow'.