Chances are about even that Emily is a sheep. Chances are slight that Fred left the football. Chances are about even that Brian is a frog. It is almost certain that if 'Fred left the football' or 'Emily is a sheep' or both then John went to the bedroom. We doubt that if either 'Brian is a frog' or 'Fred left the football' but not both then Yann is hungry. It is certain that if 'Brian is a frog and Emily is a sheep' then Jeff moved to the garden.
It is highly unlikely that 'Fred left the football' or 'Brian is a frog' or both.
There is a better than even chance that 'Fred left the football' or 'Brian is a frog' or both.
It is highly unlikely that 'Fred left the football' or 'Brian is a frog' 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
  0.5::factA.
  0.1::factB.
  0.5::factC.
  %hop1
  0.95::factX:-or(factB,factA).
  0.2::factY:-xor(factC,factB).
  1.0::factZ:-and(factC,factA).

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

  query(conclusion).
  
'Fred left the football' or 'Brian is a frog' or both.