How to use the salem.gis function in salem

lat = utils.str_in_list(vns, utils.valid_names['lat_var'])
    if (lon is None) or (lat is None):
        return None

    # OK, get it
    lon = nc.variables[lon][:]
    lat = nc.variables[lat][:]
    if len(lon.shape) != 1:
        raise RuntimeError('Coordinates not of correct shape')

    # Make the grid
    dx = lon[1]-lon[0]
    dy = lat[1]-lat[0]
    args = dict(nxny=(lon.shape[0], lat.shape[0]), proj=wgs84, dxdy=(dx, dy))
    args['corner'] = (lon[0], lat[0])
    return gis.Grid(**args)

for yp in yc:
        if len(ds.variables[yp].shape) == 1:
            y = yp
    if (x is None) or (y is None):
        return None

    # OK, get it
    x = ds.variables[x][:]
    y = ds.variables[y][:]

    # Make the grid
    dx = x[1]-x[0]
    dy = y[1]-y[0]
    args = dict(nxny=(x.shape[0], y.shape[0]), proj=proj, dxdy=(dx, dy),
                x0y0=(x[0], y[0]))
    return gis.Grid(**args)

pwrf = '+proj=lcc +lat_1={lat_1} +lat_2={lat_2} ' \
               '+lat_0={lat_0} +lon_0={lon_0} ' \
               '+x_0=0 +y_0=0 +a=6370000 +b=6370000'
        pwrf = pwrf.format(**pargs)
    elif map_proj == 'MERCATOR':
        pwrf = '+proj=merc +lat_ts={lat_1} +lon_0={lon_0} ' \
               '+x_0=0 +y_0=0 +a=6370000 +b=6370000'
        pwrf = pwrf.format(**pargs)
    elif map_proj == 'POLAR':
        pwrf = '+proj=stere +lat_ts={lat_1} +lat_0=90.0 +lon_0={lon_0} ' \
               '+x_0=0 +y_0=0 +a=6370000 +b=6370000'
        pwrf = pwrf.format(**pargs)
    else:
        raise NotImplementedError('WRF proj not implemented yet: '
                                  '{}'.format(map_proj))
    pwrf = gis.check_crs(pwrf)

    # get easting and northings from dom center (probably unnecessary here)
    e, n = gis.transform_proj(wgs84, pwrf, pargs['ref_lon'], pargs['lat_0'])

    # LL corner
    nx, ny = e_we[0]-1, e_sn[0]-1
    x0 = -(nx-1) / 2. * dx + e  # -2 because of staggered grid
    y0 = -(ny-1) / 2. * dy + n

    # parent grid
    grid = gis.Grid(nxny=(nx, ny), x0y0=(x0, y0), dxdy=(dx, dy), proj=pwrf)

    # child grids
    out = [grid]
    for ips, jps, pid, ratio, we, sn in zip(i_parent_start, j_parent_start,
                                            parent_id, parent_ratio,