  This paper presents a procedure to determine a complete belief function from
the known values of belief for some of the subsets of the frame of discerment.
The method is based on the principle of minimum commitment and a new principle
called the focusing principle. This additional principle is based on the idea
that belief is specified for the most relevant sets: the focal elements. The
resulting procedure is compared with existing methods of building complete
belief functions: the minimum specificity principle and the least commitment
principle.
