content: from halotools.empirical_models import Zheng07 , model_defaults
  from halotools.mock_observables import wp
  from halotools.mock_observables.clustering import tpcf
  from halotools.empirical_models.mock_helpers import (three_dim_pos_bundle,
                                                       infer_mask_from_kwargs)
  from halotools.mock_observables.clustering import wp
  from halotools.sim_manager import supported_sims
  import matplotlib.pyplot as plt
  plt.switch_backend("Agg")
  import time
  import numpy as np
  model = Zheng07()
  
  xir = []
  for i in range(500):
   model.populate_mock()
   xir.append(model.mock.compute_galaxy_clustering()[1])
  
  covar = np.cov(np.array(xir).T)
  np.savetxt("clustering_covariance_Mr20.dat" , covar)
  
  """
  a = time.time()
  model.mock.compute_galaxy_clustering()
  print time.time()  - a
  rbins = model_defaults.default_rbins
  rbin_centers  = (rbins[1:] + rbins[:-1])/2.
  cat = supported_sims.HaloCatalog()
  l = cat.Lbox
  print l
  p_bins = np.linspace(0,l/2,200)
  mask = infer_mask_from_kwargs(model.mock.galaxy_table)
  
  
  pos = three_dim_pos_bundle(table=model.mock.galaxy_table,
                                 key1='x', key2='y', key3='z', mask=mask,
                                 return_complement=False)
  figure = plt.figure(figsize=(10,10))
  
  cl = wp(pos , rbins, p_bins , period = l , estimator = 'Landy-Szalay')
  
  for n_pbins in np.array([2,8,16]):
  
    p_bins = np.linspace(0 , l/2 , n_pbins)
    a = time.time()
    clustering = wp(pos, rbins, p_bins , period = l , estimator = 'Landy-Szalay')
    print time.time() - a
    plt.plot(rbin_centers , (clustering)/cl , label = "$N\pi_{bin}$="+str(n_pbins) , lw = 2)
    plt.xscale("Log")
    plt.yscale("Log")
    plt.legend() 
  plt.savefig("/home/mj/public_html/wpex.png")"""
  