  We prove a Darboux theorem for formal deformations of Hamiltonian operators
of hydrodynamic type (Dubrovin-Novikov). Not all deformations are equivalent to
the original operator: there is a moduli 2-stack of normal forms.
  The paper utilizes three main concepts:
  1) dg Lie algebras concentrated in degrees [-1,\infty) such as the Schouten
algebra - these give a convenient language for describing deformation problems;
  2) the Deligne 2-groupoid associated to such a dg Lie algebra, which
represents the moduli of formal deformations;
  3) a refined version of the Schouten bracket in the formal calculus of
variations, due to V. O. Soloviev (hep-th/9305133).
