  We study a model of machine teaching where the teacher mapping is constructed
from a size function on both concepts and examples. The main question in
machine teaching is the minimum number of examples needed for any concept, the
so-called teaching dimension. A recent paper [7] conjectured that the worst
case for this model, as a function of the size of the concept class, occurs
when the consistency matrix contains the binary representations of numbers from
zero and up. In this paper we prove their conjecture. The result can be seen as
a generalization of a theorem resolving the edge isoperimetry problem for
hypercubes [12], and our proof is based on a lemma of [10].
