Chances are about even that John left the milk. Chances are about even that Julius is white. There is a very good chance that Lily is a rhino. There is almost no chance that if 'Julius is white and John left the milk' then Mary got the football. Chances are about even that if either 'Lily is a rhino' or 'Julius is white' but not both then Brian is yellow. It is highly unlikely that if either 'John left the milk' or 'Lily is a rhino' but not both then Greg is a swan.
There is a very good chance that either 'Greg is a swan' or 'Lily is a rhino' but not both.
There is a very good chance that either 'Greg is a swan' or 'Lily is a rhino' but not both.
It is impossible that either 'Greg is a swan' or 'Lily is a rhino' but not 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
  0.5::factA.
  0.5::factB.
  0.8::factC.
  %hop1
  0.02::factX:-and(factB,factA).
  0.5::factY:-xor(factC,factB).
  0.05::factZ:-xor(factA,factC).

  %hop2
  conclusion:-xor(factZ,factC).

  query(conclusion).
  
Either 'Greg is a swan' or 'Lily is a rhino' but not both.