class_docstr: A generic link function for one-parameter exponential
  family, with call, inverse and deriv methods.
code: def deriv(self, p):
      return NotImplementedError
  
imports: import numpy
  import scipy.stats
label_desc: Use image node_id 4 for calling the Link obj's underlying member method code with example usage: obj.deriv(p) and returns: NotImplementedError