We believe that Bernhard is a swan. Chances are about even that Greg is white. It is unlikely that John discarded the milk. Chances are slight that if 'Greg is white and Bernhard is a swan' then Julius is a frog. We doubt that if 'Greg is white and Bernhard is a swan' then Mary picked up the apple. It is improbable that if 'Greg is white and Bernhard is a swan' then Brian is white.
Chances are about even that 'Brian is white' or 'Greg is white' or both.
Chances are about even that 'Brian is white' or 'Greg is white' or both.
There is almost no chance that 'Brian is white' or 'Greg is white' 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
  0.75::factA.
  0.5::factB.
  0.2::factC.
  %hop1
  0.1::factX:-and(factB,factA).
  0.2::factY:-and(factB,factA).
  0.1::factZ:-and(factB,factA).

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

  query(conclusion).
  
'Brian is white' or 'Greg is white' or both.