  We extend the notion of belief function to the case where the underlying
structure is no more the Boolean lattice of subsets of some universal set, but
any lattice, which we will endow with a minimal set of properties according to
our needs. We show that all classical constructions and definitions (e.g., mass
allocation, commonality function, plausibility functions, necessity measures
with nested focal elements, possibility distributions, Dempster rule of
combination, decomposition w.r.t. simple support functions, etc.) remain valid
in this general setting. Moreover, our proof of decomposition of belief
functions into simple support functions is much simpler and general than the
original one by Shafer.
