  Some general properties of compatible Poisson brackets of hydrodynamic type
are discussed, in particular:
  (1) an invariant differential-geometric criterion of the compatibility based
on the Nijenhuis tensor;
  (2) the Lax pair with a spectral parameter governing compatible Poisson
brackets in the diagonalizable case;
  (3) the connection of this problem with the class of surfaces in Euclidean
space which possess nontrivial deformations preserving the Weingarten operator.
