It is highly likely that Gertrude is a mouse. It is improbable that John dropped the apple. Chances are slight that Julius is a swan. It is highly unlikely that if 'John dropped the apple and Gertrude is a mouse' then Brian is green. Chances are about even that if 'Julius is a swan and Gertrude is a mouse' then Lily is a frog. It is certain that if either 'Julius is a swan' or 'Gertrude is a mouse' but not both then Mary went to the office.
There is a very good chance that 'Brian is green' or 'Mary went to the office' or both.
There is a very good chance that 'Brian is green' or 'Mary went to the office' or both.
We doubt that 'Brian is green' or 'Mary went to the office' 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
  0.9::factA.
  0.1::factB.
  0.1::factC.
  %hop1
  0.05::factX:-and(factB,factA).
  0.5::factY:-and(factC,factA).
  1.0::factZ:-xor(factC,factA).

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

  query(conclusion).
  
'Brian is green' or 'Mary went to the office' or both.