  It is shown that the main variable Z of the Null Surface Formulation of GR is
the generating function of a constrained Lagrange submanifold that lives on the
energy surface H=0 and that its level surfaces Z=const. are Legendre
submanifolds on that energy surface.
  The behaviour of the variable Z at the caustic points is analysed and a
genralization of this variable is discussed.
