  Given a well-ordered semi-group $\Gamma$ with a minimal system of generators
of ordinal type at most $\omega n$ and of rational rank $r$, which satisfies a
positivity and increasing condition, we construct a zero-dimensional valuation
centered on the ring of polynomials with $r$ variables such that the semi-group
of the values of the polynomial ring is equal to $\Gamma$. The construction
uses a generalization of Favre and Jonsson's version of MacLane's sequence of
key-polynomials.
