  Inappropriate use of Dempster's rule of combination has led some authors to
reject the Dempster-Shafer model, arguing that it leads to supposedly
unacceptable conclusions when defaults are involved. A most classic example is
about the penguin Tweety. This paper will successively present: the origin of
the miss-management of the Tweety example; two types of default; the correct
solution for both types based on the transferable belief model (our
interpretation of the Dempster-Shafer model (Shafer 1976, Smets 1988)); Except
when explicitly stated, all belief functions used in this paper are simple
support functions, i.e. belief functions for which only one proposition (the
focus) of the frame of discernment receives a positive basic belief mass with
the remaining mass being given to the tautology. Each belief function will be
described by its focus and the weight of the focus (e.g. m(A)=.9). Computation
of the basic belief masses are always performed by vacuously extending each
belief function to the product space built from all variables involved,
combining them on that space by Dempster's rule of combination, and projecting
the result to the space corresponding to each individual variable.
