def add_indep(x, varnames, dtype=None):
    #TODO: this needs tests for subclasses

    if isinstance(x, np.ndarray) and x.ndim == 2:
        x = x.T

    nvars_orig = len(x)
    nobs = len(x[0])
    #print('nobs, nvars_orig', nobs, nvars_orig)
    if not dtype:
        dtype = np.asarray(x[0]).dtype
    xout = np.zeros((nobs, nvars_orig), dtype=dtype)
    count = 0
    rank_old = 0
    varnames_new = []
    varnames_dropped = []
    keepindx = []
    for (xi, ni) in zip(x, varnames):
        #print(xi.shape, xout.shape)
        xout[:,count] = xi
        rank_new = np_matrix_rank(xout)
        #print(rank_new)
        if  rank_new > rank_old:
            varnames_new.append(ni)
            rank_old = rank_new
            count += 1
        else:
            varnames_dropped.append(ni)

    return xout[:,:count], varnames_new
construct array with independent columns
dataset/ETHPy150Open statsmodels/statsmodels/statsmodels/tools/catadd.py/add_indep