It is highly unlikely that Mary got the football. It is impossible that Sandra grabbed the apple. We believe that Brian is a swan. We doubt that if 'Mary got the football' or 'Sandra grabbed the apple' or both then Winona is a mouse. There is a very good chance that if 'Brian is a swan and Mary got the football' then John went to the office. Chances are slight that if 'Mary got the football and Brian is a swan' then Lily is a frog.
It is highly unlikely that either 'Mary got the football' or 'Lily is a frog' but not both.
It is highly unlikely that either 'Mary got the football' or 'Lily is a frog' but not both.
It is probable that either 'Mary got the football' or 'Lily is a frog' 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
  0.05::factA.
  0.0::factB.
  0.75::factC.
  %hop1
  0.2::factX:-or(factA,factB).
  0.8::factY:-and(factC,factA).
  0.1::factZ:-and(factA,factC).

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

  query(conclusion).
  
Either 'Mary got the football' or 'Lily is a frog' but not both.