code: def do_postfix(self, arg):
          
          usable, filename, append = self._redirect_split(arg)
          sargs = usable.split()
          if len(sargs) == 1 and sargs[0] == "list":
              self._print_map_dict("functions", filename, append)
          elif len(sargs) >= 2:
              defvars = self._postfix_varlist("postfix " + arg)
              for var in defvars.values():
                  if not self._validate_var(var):
                      msg.err("Variable '{}' is not a valid variable|property combination.")
                      return
  
              fxn = arg.split()[-1]
              if ":" not in fxn:
                  msg.err("{} is not a valid postfix function expression.")
                  self.help_postfix()
                  return
  
              modvar = fxn.split(":")[0]
              if modvar not in defvars:
                  msg.err("Invalid postfix function: variable '{}' not defined.".format(modvar))
                  return
              
              defvars["lambda"] = fxn
              self.curargs["functions"][defvars[modvar]] = defvars
              #Give the user some feedback so that they know it was successful.
              self.do_postfix("list")
docs: Sets the function to apply to the values of a specific variable before plotting
          or tabulating values.