code: def mean_func(x):
      """mean function for example"""
      return x + 0.25 * x**2
  
file_path: statsmodels/statsmodels/robust/tests/test_mquantiles.py
imports: import numpy
  from numpy.testing import assert_allclose
  import pandas
  from statsmodels.regression.linear_model import OLS
  from statsmodels.regression.quantile_regression import QuantReg
  from statsmodels.robust import norms
  from statsmodels.robust.robust_linear_model import RLM
label_desc: Use image node_id 1 for calling a global function with example usage: mean_func(x) and returns: unknown