code: def _external2internal_lambda(bound):
      
      lower, upper = bound
      
      if lower is None and upper is None: # no constraints
          return lambda x: x
      elif upper is None:     # only lower bound
          return lambda x: sqrt((x - lower + 1.)**2 - 1)
      elif lower is None:     # only upper bound
          return lambda x: sqrt((x - upper + 1.)**2 - 1)
      else:
          return lambda x: arcsin((2. * (x - lower) / (upper - lower)) - 1.)
docs: Make a lambda function which converts an single external (constained)
      parameter to a internal (uncontrained) parameter.