  Consider a linear space L of complex D-dimensional linear operators, and
assume that some power L^k of L is the whole space of DxD matrices.
Perez-Garcia, Verstraete, Wolf and Cirac conjectured that the sequence
L^1,L^2,... stablilizes after O(D^2) terms; we prove that this happens after
O(D^2 log(D)) terms, improving the previously known bound of O(D^4).
