It is likely that Mary dropped the milk. There is a very good chance that Julius is a frog. It is probably the case that Lily is a swan. It is probably not the case that if 'Julius is a frog' or 'Mary dropped the milk' or both then John moved to the office. We doubt that if 'Julius is a frog' or 'Lily is a swan' or both then Emily is a mouse. It is almost certain that if 'Mary dropped the milk' or 'Julius is a frog' or both then Greg is a lion.
It is highly unlikely that either 'Julius is a frog' or 'Lily is a swan' but not both.
Chances are about even that either 'Julius is a frog' or 'Lily is a swan' but not both.
It is highly unlikely that either 'Julius is a frog' or 'Lily 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.
  0.8::factB.
  0.7::factC.
  %hop1
  0.25::factX:-or(factB,factA).
  0.2::factY:-or(factB,factC).
  0.95::factZ:-or(factA,factB).

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

  query(conclusion).
  
Either 'Julius is a frog' or 'Lily is a swan' but not both.