def remove_constant_features(sfm):
    """
    Remove features that are constant across all samples
    """
    # boolean matrix of whether x == first column (feature)
    x_not_equal_to_1st_row = sfm._x != sfm._x[0]
    non_const_f_bool_ind = x_not_equal_to_1st_row.sum(axis=0) >= 1
    return sfm.ind_x(selected_f_inds=non_const_f_bool_ind)