  We give a new characterization of elementary and deterministic polynomial
time computation in linear logic through the proofs-as-programs correspondence.
Girard's seminal results, concerning elementary and light linear logic, achieve
this characterization by enforcing a stratification principle on proofs, using
the notion of depth in proof nets. Here, we propose a more general form of
stratification, based on inducing levels in proof nets by means of indexes,
which allows us to extend Girard's systems while keeping the same complexity
properties. In particular, it turns out that Girard's systems can be recovered
by forcing depth and level to coincide. A consequence of the higher flexibility
of levels with respect to depth is the absence of boxes for handling the
paragraph modality. We use this fact to propose a variant of our polytime
system in which the paragraph modality is only allowed on atoms, and which may
thus serve as a basis for developing lambda-calculus type assignment systems
with more efficient typing algorithms than existing ones.
