class_docstr: asymmetric negative logistic model of Joe 1990
  
  special case:  a1=a2=1 : symmetric negative logistic of Galambos 1978
  
  restrictions:
   - theta in (0,inf)
   - a1, a2 in (0,1]
code: def deriv2(self, t, a1, a2, theta):
      b = theta
      a1, a2 = a2, a1
      a1tp = (a1 * (1 - t)) ** (1 / b)
      a2tp = (a2 * t) ** (1 / b)
      a1tn = (a1 * (1 - t)) ** (-1 / b)
      a2tn = (a2 * t) ** (-1 / b)
  
      t1 = (b + 1) * a2tp * a1tp * (a1tn + a2tn) ** (-b)
      t2 = b * (1 - t) ** 2 * t**2 * (a1tp + a2tp) ** 2
      d2 = t1 / t2
      return d2
  
imports: import numpy
  from scipy import stats
  from statsmodels.tools.numdiff import _approx_fprime_cs_scalar, approx_hess
label_desc: Use image node_id 4 for calling the AsymNegLogistic obj's underlying member method code with example usage: obj.deriv2(t, a1, a2, theta) and returns: d2