code: def link(self, parameter_1, parameter_2, link_function=None):
          
          if not isinstance(parameter_1,list):
          # Make a list of one element
              parameter_1_list  = [parameter_1]
          else:
          # Make a copy to avoid tampering with the input
              parameter_1_list = list(parameter_1)
          
          
          for param_1 in parameter_1_list:
              assert param_1.path in self, "Parameter %s is not contained in this model" % param_1.path
  
          assert parameter_2.path in self, "Parameter %s is not contained in this model" % parameter_2.path
          
          if link_function is None:
              # Use the Line function by default, with both parameters fixed so that the two
              # parameters to be linked will vary together
              link_function = get_function('Line')
  
              link_function.a.value = 1
              link_function.a.fix = True
  
              link_function.b.value = 0
              link_function.b.fix = True
          
          
          
          for param_1 in parameter_1_list: 
              param_1.add_auxiliary_variable(parameter_2, link_function)
              # Now set the units of the link function
              link_function.set_units(parameter_2.unit, param_1.unit)
docs: Link the value of the provided parameters through the provided function (identity is the default, i.e.,
          parameter_1 = parameter_2).
  
          :param parameter_1: the first parameter;can be either a single parameter or a list of prarameters
          :param parameter_2: the second parameter
          :param link_function: a function instance. If not provided, the identity function will be used by default.
          Otherwise, this link will be set: parameter_1 = link_function(parameter_2)
          :return: (none)