There is a very good chance that Jeff went to the hallway. Chances are slight that Jessica is a sheep. We doubt that Julius is a rhino. Chances are slight that if either 'Julius is a rhino' or 'Jeff went to the hallway' but not both then Greg is white. It is highly unlikely that if either 'Jessica is a sheep' or 'Julius is a rhino' but not both then Mary dropped the milk. It is probably the case that if either 'Jeff went to the hallway' or 'Jessica is a sheep' but not both then Bernhard is a swan.
There is little chance that 'Jeff went to the hallway' or 'Mary dropped the milk' or both.
There is a very good chance that 'Jeff went to the hallway' or 'Mary dropped the milk' or both.
There is little chance that 'Jeff went to the hallway' or 'Mary dropped the milk' 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
  0.8::factA.
  0.1::factB.
  0.2::factC.
  %hop1
  0.1::factX:-xor(factC,factA).
  0.05::factY:-xor(factB,factC).
  0.7::factZ:-xor(factA,factB).

  %hop2
  conclusion:-or(factA,factY).

  query(conclusion).
  
'Jeff went to the hallway' or 'Mary dropped the milk' or both.