<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def iter_fit_shifts(xy,uv,nclip=3,sigma=3.0):
 """ Perform an iterative-fit with 'nclip' iterations """

    fit = fit_shifts(xy,uv)
    if nclip is None: nclip = 0
    # define index to initially include all points
    for n in range(nclip):
        resids = compute_resids(xy,uv,fit)
        resids1d = np.sqrt(np.power(resids[:,0],2)+np.power(resids[:,1],2))
        sig = resids1d.std()
        # redefine what pixels will be included in next iteration
        goodpix = resids1d < sigma*sig
        xy = xy[goodpix]
        uv = uv[goodpix]
        fit = fit_shifts(xy,uv)

    fit['img_coords'] = xy
    fit['ref_coords'] = uv

    return fit