static void replace_in_grid(int x1, int y1, int x2, int y2,
                            int oldfeat, int newfeat)
{
    for ( int x = x1; x < x2; ++x )
        for ( int y = y1; y < y2; ++y )
            if ( grd[x][y] == oldfeat )
                grd[x][y] = newfeat;
}

    // XXX TODO: fractalisation

    // Fractalisation
    // LAVA is a placeholder for cells which will become shallow water
    // at the end of the current iteration.
    // WATER_STUCK is a placeholder for last iteration's generated water.
    replace_in_grid(margin, margin, GXM-margin, GYM-margin,
                    DNGN_SHALLOW_WATER, DNGN_WATER_STUCK);
    for ( int i = 0; i < 6; ++i )
    {
        for ( int x = margin; x < GXM - margin; ++x )
        {
            for ( int y = margin; y < GYM - margin; ++y )
            {
                if ( grd[x][y] == DNGN_DEEP_WATER )
                {
                    int badness = count_neighbours(x, y, DNGN_WATER_STUCK);
                    if ( random2(badness) >= 2 && coinflip() )
                        grd[x][y] = DNGN_LAVA;
                }
            }
        }
        replace_in_grid(margin, margin, GXM-margin, GYM-margin,
                        DNGN_LAVA, DNGN_WATER_STUCK);
    }
    replace_in_grid(margin, margin, GXM-margin, GYM-margin,
                    DNGN_WATER_STUCK, DNGN_SHALLOW_WATER);