  The main scope of this short paper is to provide a modification of the axioms
given by Messmer and Wood for the theory of separably closed fields of positive
characteristic and finite imperfectness degree. The original axioms failed to
meet natural expectations, and therefore a new axiomatization was given (i.e.
Ziegler's one), but the new axioms do not follow the initial idea of Messmer
and Wood. Therefore, we aim to give a correct axiomatization which is more
similar to the original one and which, as the original axioms,involves only one
Hasse-Schmidt derivation, this time based on the iterativity conditions
corresponding to the Witt group.
