class_docstr: model of Huesler Reiss 1989
  
  special case:  a1=a2=1 : symmetric negative logistic of Galambos 1978
  
  restrictions:
   - lambda in (0,inf)
code: def deriv2(self, t, lamda):
      return self._derivs(t, lamda, 2)
  
imports: import numpy
  from scipy import stats
  from statsmodels.tools.numdiff import _approx_fprime_cs_scalar, approx_hess
label_desc: Use image node_id 5 for calling the HR obj's underlying member method code with example usage: obj.deriv2(t, lamda) and returns: self