Makes sure that the command specified in the line is valid given the current
        status of loaded unit tests and analysis group.

def precmd(self, line):
        """Makes sure that the command specified in the line is valid given the current
        status of loaded unit tests and analysis group.
        """
        if line == "":
            return ""
        command = line.split()[0]
        if "!" in command:
            value = command.split("!")[1]
            try:
                ihist = int(value)
                import readline
                if ihist <= readline.get_current_history_length():
                    return readline.get_history_item(ihist)
                else:
                    msg.warn("The specified history item {} ".format(ihist) + 
                             "does not exist in the history.")
                    return ""
            except ValueError:
                #This clearly isn't a bash style recursion of a past history item.
                #Just run the command as it was originally entered.
                return line
        else:
            if command in self._test_cmds or command in self._group_cmds:
                #We have to test the active unit test for both the test commands and the
                #group commands, since the group commands rely on the active unit test.
                if self.active is None or self.active not in self.tests:
                    msg.err("The specified command '{}' requires an ".format(command) + 
                            "active unit test to be loaded. Use 'parse' or 'load'.")
                    return ""
                elif (command in self._group_cmds and (self.group is None or 
                                                       self.group not in self.args[self.active])):
                    msg.err("No valid analysis group is active. Use 'group' to create " 
                            "one or mark an existing one as active.")
                    return ""
                else:
                    return line

            if command in self._var_cmds:
                #We need to make sure that we have variables set.
                if self.curargs["independent"] is None or len(self.curargs["dependents"]) == 0:
                    msg.err("This command requires an independent variable to be set and "
                            "at least one dependent variable.\n See 'dep' and 'indep' commands.")
                    return ""
                else:
                    return line
            else:
                return line