def initialDataLists(D):
  """ Initialize grid, scalars, and vectors lists used in convertToVtk function. """
  grid = []; # simple
  scalars = [list(x) for x in [()]*len(D["scalar"].keys())]; # more complicated
  vectors = [list(x) for x in [()]*len(D["vector"].keys())]; # more complicated
  return [grid, scalars, vectors];