It is highly unlikely that Bill left the football. We doubt that Emily is a cat. We believe that John went to the garden. It is likely that if either 'Emily is a cat' or 'John went to the garden' but not both then Sandra dropped the milk. It is probably not the case that if 'Emily is a cat' or 'Bill left the football' or both then Brian is a lion. It is probable that if 'John went to the garden' or 'Emily is a cat' or both then Lily is a swan.
Chances are about even that 'Bill left the football' or 'Sandra dropped the milk' or both.
Chances are about even that 'Bill left the football' or 'Sandra dropped the milk' or both.
It is impossible that 'Bill left the football' or 'Sandra dropped the milk' or 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.
  0.2::factB.
  0.75::factC.
  %hop1
  0.7::factX:-xor(factB,factC).
  0.25::factY:-or(factB,factA).
  0.7::factZ:-or(factC,factB).

  %hop2
  conclusion:-or(factA,factX).

  query(conclusion).
  
'Bill left the football' or 'Sandra dropped the milk' or both.