It is probably the case that Mary went to the bedroom. Chances are about even that Brian is a rhino. It is probably the case that Lily is a lion. We doubt that if 'Mary went to the bedroom' or 'Lily is a lion' or both then Greg is a swan. It is certain that if either 'Lily is a lion' or 'Brian is a rhino' but not both then Gertrude is a sheep. It is likely that if 'Lily is a lion and Brian is a rhino' then Sandra dropped the milk.
It is impossible that either 'Sandra dropped the milk' or 'Lily is a lion' but not both.
Chances are about even that either 'Sandra dropped the milk' or 'Lily is a lion' but not both.
It is impossible that either 'Sandra dropped the milk' or 'Lily is a lion' 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.
  0.5::factB.
  0.7::factC.
  %hop1
  0.2::factX:-or(factA,factC).
  1.0::factY:-xor(factC,factB).
  0.7::factZ:-and(factC,factB).

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

  query(conclusion).
  
Either 'Sandra dropped the milk' or 'Lily is a lion' but not both.