  A map $\phi:M_m(\bC)\to M_n(\bC)$ is decomposable if it is of the form
$\phi=\phi_1+\phi_2$ where $\phi_1$ is a CP map while $\phi_2$ is a co-CP map.
It is known that if $m=n=2$ then every positive map is decomposable. Given an
extremal unital positive map $\phi:M_2(\bC)\to M_2(\bC)$ we construct concrete
maps (not necessarily unital) $\phi_1$ and $\phi_2$ which give a decomposition
of $\phi$. We also show that in most cases this decomposition is unique.
