If f\colon M\to\R^3 is a tight immersion, then
M can be decomposed into two regions, M^+ and M^ where
 K\ge 0 on M^+ and
K\le 0 on M^
 f embeds M^+ onto
the complement in \dHf of a finite number of
disjoint planar convex closed disks
D_1,\ldots,D_k in \dHf
 for each i,
f^{1}(\partial D_i) is a nontrivial 1cycle in M
(these are called top cycles)
