We doubt that Bernhard is gray. There is little chance that Sandra left the milk. It is impossible that Greg is a rhino. Chances are about even that if either 'Sandra left the milk' or 'Greg is a rhino' but not both then Gertrude is a wolf. It is impossible that if 'Greg is a rhino and Bernhard is gray' then Brian is a lion. There is a very good chance that if either 'Greg is a rhino' or 'Sandra left the milk' but not both then Lily is a rhino.
It is impossible that 'Brian is a lion and Greg is a rhino'.
It is impossible that 'Brian is a lion and Greg is a rhino'.
There is a very good chance that 'Brian is a lion and Greg is a rhino'.

  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
  0.2::factA.
  0.1::factB.
  0.0::factC.
  %hop1
  0.5::factX:-xor(factB,factC).
  0.0::factY:-and(factC,factA).
  0.8::factZ:-xor(factC,factB).

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

  query(conclusion).
  
'Brian is a lion and Greg is a rhino'.