  For a finite valued field extension $(L/K,v)$ we describe the problem of find
sets of generators for the corresponding extension $\mathcal O_L/\mathcal O_K$
of valuation rings. The main tool to obtain such sets are complete sets of
(key) polynomials. We show that when the initial index coincide with the
ramification index, sequences of key polynomials naturally give rise to sets of
generators. We use this to prove Knaf's conjecture for pure extensions.
