code: def vech(A):
      """
      Simple vech operator
      Returns
      -------
      vechvec: vector of all elements on and below diagonal
      """
  
      length = A.shape[1]
      vechvec = []
      for i in range(length):
          b = i
          while b < length:
              vechvec.append(A[b, i])
              b = b + 1
      vechvec = np.asarray(vechvec)
      return vechvec
  
file_path: statsmodels/statsmodels/tsa/vector_ar/util.py
imports: from statsmodels.compat.pandas import frequencies
  from statsmodels.compat.python import asbytes
  from statsmodels.tools.validation import array_like, int_like
  import numpy
  import pandas
  from scipy import stats, linalg
  import statsmodels.tsa.tsatools
label_desc: Use image node_id 11 for calling a global function with example usage: vech(A) and returns: vechvec