It is impossible that Sandra left the football. It is highly unlikely that Jeff went to the bedroom. There is a very good chance that Greg is a frog. It is highly unlikely that if 'Greg is a frog and Jeff went to the bedroom' then Bernhard is gray. There is little chance that if 'Greg is a frog and Jeff went to the bedroom' then Julius is gray. There is a better than even chance that if either 'Sandra left the football' or 'Greg is a frog' but not both then Brian is a swan.
It is impossible that 'Brian is a swan' or 'Jeff went to the bedroom' or both.
Chances are about even that 'Brian is a swan' or 'Jeff went to the bedroom' or both.
It is impossible that 'Brian is a swan' or 'Jeff went to the bedroom' 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
  0.0::factA.
  0.05::factB.
  0.8::factC.
  %hop1
  0.05::factX:-and(factC,factB).
  0.1::factY:-and(factC,factB).
  0.6::factZ:-xor(factA,factC).

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

  query(conclusion).
  
'Brian is a swan' or 'Jeff went to the bedroom' or both.