Chances are slight that Mary dropped the milk. It is highly likely that Brian is a rhino. There is a better than even chance that Emily is a sheep. It is probably the case that if either 'Brian is a rhino' or 'Emily is a sheep' but not both then Julius is yellow. It is probable that if either 'Mary dropped the milk' or 'Brian is a rhino' but not both then John grabbed the apple. It is probably not the case that if 'Emily is a sheep and Brian is a rhino' then Greg is a swan.
Chances are about even that either 'Greg is a swan' or 'Julius is yellow' but not both.
Chances are about even that either 'Greg is a swan' or 'Julius is yellow' but not both.
It is impossible that either 'Greg is a swan' or 'Julius is yellow' 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
  0.1::factA.
  0.9::factB.
  0.6::factC.
  %hop1
  0.7::factX:-xor(factB,factC).
  0.7::factY:-xor(factA,factB).
  0.25::factZ:-and(factC,factB).

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

  query(conclusion).
  
Either 'Greg is a swan' or 'Julius is yellow' but not both.