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 deriv(self, t, a1, a2, theta):
      a1, a2 = a2, a1
      m1 = -1 / theta
      m2 = m1 - 1
  
      # (a1^(-1/θ) (1 - t)^(-1/θ - 1) - a2^(-1/θ) t^(-1/θ - 1))*
      # (a1^(-1/θ) (1 - t)^(-1/θ) + (a2 t)^(-1/θ))^(-θ - 1)
  
      d1 = (a1**m1 * (1 - t) ** m2 - a2**m1 * t**m2) * (
          (a1 * (1 - t)) ** m1 + (a2 * t) ** m1
      ) ** (-theta - 1)
      return d1
  
imports: import numpy
  from scipy import stats
  from statsmodels.tools.numdiff import _approx_fprime_cs_scalar, approx_hess
label_desc: Use image node_id 3 for calling the AsymNegLogistic obj's underlying member method code with example usage: obj.deriv(t, a1, a2, theta) and returns: d1