It is probable that Sandra dropped the milk. It is highly unlikely that Lily is a rhino. There is a better than even chance that Brian is a swan. We doubt that if 'Sandra dropped the milk and Lily is a rhino' then John moved to the garden. There is a very good chance that if 'Brian is a swan' or 'Lily is a rhino' or both then Greg is green. It is probably not the case that if 'Lily is a rhino and Sandra dropped the milk' then Mary discarded the milk.
It is highly unlikely that either 'Mary discarded the milk' or 'Brian is a swan' but not both.
There is a better than even chance that either 'Mary discarded the milk' or 'Brian is a swan' but not both.
It is highly unlikely that either 'Mary discarded the milk' or 'Brian is a swan' 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
  0.7::factA.
  0.05::factB.
  0.6::factC.
  %hop1
  0.2::factX:-and(factA,factB).
  0.8::factY:-or(factC,factB).
  0.25::factZ:-and(factB,factA).

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

  query(conclusion).
  
Either 'Mary discarded the milk' or 'Brian is a swan' but not both.