We doubt that Brian is a rhino. It is highly likely that Bernhard is a frog. Chances are slight that Jeff left the apple. Chances are about even that if either 'Jeff left the apple' or 'Brian is a rhino' but not both then John went to the garden. It is probably not the case that if 'Jeff left the apple and Brian is a rhino' then Mary dropped the milk. It is highly unlikely that if 'Brian is a rhino' or 'Jeff left the apple' or both then Julius is yellow.
It is impossible that 'Mary dropped the milk and Bernhard is a frog'.
It is impossible that 'Mary dropped the milk and Bernhard is a frog'.
There is a very good chance that 'Mary dropped the milk and Bernhard is a frog'.

  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
  0.2::factA.
  0.9::factB.
  0.1::factC.
  %hop1
  0.5::factX:-xor(factC,factA).
  0.25::factY:-and(factC,factA).
  0.05::factZ:-or(factA,factC).

  %hop2
  conclusion:-and(factY,factB).

  query(conclusion).
  
'Mary dropped the milk and Bernhard is a frog'.