  The aim of the paper is to show that the nowadays experimental data from
superallowed nuclear and neutron $\beta$ decays, and from leptonic and
semileptonic decays allow the finding of the most probable numerical form of
the KM matrix, as well as the determination of decay constants, $f_{P}$, and of
various form factors $f_+(q^2)$, by using a genuine implementation of unitarity
constraints. In particular this approach allows the determination of
semileptonic form factors that is illustrated on the existing data of
$D\rightarrow \pi l \nu$ and $D\rightarrow K l\nu$ decays.
