It is impossible that John dropped the apple. It is certain that Greg is gray. There is a very good chance that Julius is a swan. Chances are about even that if 'Julius is a swan' or 'Greg is gray' or both then Brian is a rhino. It is likely that if 'John dropped the apple and Greg is gray' then Lily is white. We believe that if 'Julius is a swan' or 'Greg is gray' or both then Mary went to the office.
It is impossible that 'Julius is a swan and John dropped the apple'.
It is impossible that 'Julius is a swan and John dropped the apple'.
There is a very good chance that 'Julius is a swan and John dropped the apple'.

  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
  0.0::factA.
  1.0::factB.
  0.8::factC.
  %hop1
  0.5::factX:-or(factC,factB).
  0.7::factY:-and(factA,factB).
  0.75::factZ:-or(factC,factB).

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

  query(conclusion).
  
'Julius is a swan and John dropped the apple'.