docstr: trace of hat matrix
          
docstr_tok: ["trace","of","hat","matrix"]
code: def hatmatrix_trace(self):
      """trace of hat matrix"""
      return self.hatmatrix_diag.sum()
  
imports: from statsmodels.compat.python import lrange
  import numpy
  from statsmodels.tools.decorators import cache_readonly
  from statsmodels.regression.linear_model import OLS, GLS, RegressionResults
  from statsmodels.regression.feasible_gls import atleast_2dcols
label_desc: Use image node_id 3 for calling the TheilRegressionResults obj's underlying member method code with example usage: obj.hatmatrix_trace() and returns: self