Before revision:  First, we establish in full generality the equivalence between the extremality of a given measure Q and the denseness in L^1(Q) of a suitable linear subspace, which can be seen  as the set of all semi-static trading strategies.
After revision:  First, we establish in full generality the equivalence between the extremality of a given measure Q and the denseness in L^1(Q) of a suitable linear subspace, which can be seen in a financial context as the set of all semi-static trading strategies.