  In this article, we discuss a new software package of kinematic and vertex
fitting for the CMD-3 experiment at the VEPP-2000 electron-positron collider.
The authors describe in detail the fitting algorithm, parametrization of
four-momenta and trajectories of various particles and present the results of
testing the fitting package using events of Monte Carlo simulation of various
$e^+e^-$ annihilation processes. The authors also provide several examples of
the fitting package validation using Gaussian simulation. Although the package
discussed in this article is intended for the CMD-3 experiment, it can also be
used in other experiments. The authors consider the described package as their
first step towards a more universal and rigrous kinematic and vertex fitting
package that can be used in future $e^+e^-$ experiments, such as the Super
Charm-Tau factory.
