  We discuss the self-consistency imposed by the analyticity of regular parts
of form factors, appearing in the $z$-expansion for semileptonic $B$-meson
decays, when fitted in different kinematic regions. Relying on the uniqueness
of functions defined by analytic continuation, we propose four metrics which
measure the departure from the ideal analytic self-consistency. We illustrate
the process using Belle data for $B\rightarrow D\ell \nu_\ell$. For this
specific example, the metrics provide consistent indications that some choices
(order of truncation, BGL or BCL) made in the form of the $z$-expansion can be
optimized. However, other choices ($z$-origin, location of isolated poles and
threshold constraints) appear to have very little effect on these metrics. We
briefly discuss the implication for optimization of the $z$-expansion for
nucleon form factors relevant for neutrino oscillation experiments.
