  Discrete Lagrangian Systems on graphs are considered. Vector-valued closed
differential 2-form on the space of solutions is constructed. This form takes
values in the first homology group of the graph. This construction generalizes
the Symplectic Wronskian for the linear self-adjoint operators on graphs found
in 1997 by the first author and used for the needs of the Scattering Theory for
graphs with tails
