One question about the validation error message

[eluser]luffy[/eluser]
The code below:

$this->form_validation->set_rules('username', 'username', 'required');

$this->form_validation->set_rules('content', 'content', 'required');

$this->form_validation->set_message('required', 'custom message');

For example:
If I want to show the "username error" and "content error"
How to use the set_message?
Because their rule is the same.