  If two distance-3 codes have the same neighborhood, then each of them is
called a mobile set. In the (4k+3)-dimensional binary hypercube, there exists a
mobile set of cardinality 2*6^k that cannot be split into mobile sets of
smaller cardinalities or represented as a natural extension of a mobile set in
a hypercube of smaller dimension. Keywords: mobile set; 1-perfect code.
