It is certain that Mary dropped the milk. It is impossible that Emily is a cat. It is improbable that Bernhard is gray. It is probably not the case that if 'Mary dropped the milk' or 'Emily is a cat' or both then Lily is green. There is a better than even chance that if either 'Mary dropped the milk' or 'Emily is a cat' but not both then Gertrude is a wolf. Chances are about even that if 'Mary dropped the milk' or 'Bernhard is gray' or both then Brian is a rhino.
There is little chance that 'Bernhard is gray' or 'Emily is a cat' or both.
There is little chance that 'Bernhard is gray' or 'Emily is a cat' or both.
There is a very good chance that 'Bernhard is gray' or 'Emily is a cat' 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
  1.0::factA.
  0.0::factB.
  0.1::factC.
  %hop1
  0.25::factX:-or(factA,factB).
  0.6::factY:-xor(factA,factB).
  0.5::factZ:-or(factA,factC).

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

  query(conclusion).
  
'Bernhard is gray' or 'Emily is a cat' or both.