Please, look:
http://pedro-3.1104361.n5.nabble.com/template/NamlServlet.jtp?macro=login_page&nextUrl=http%3A%2F%2Fpedro-3.1104361.n5.nabble.com%2FCode:
<override_macro name="login_page" requires="servlet">
<n.set_var. name="error"><n.false/></n.set_var.>
<n.if.is_submitted_form>
<then>
<n.do_login>
<email><n.email_field.value/></email>
<password><n.password_field.value/></password>
<nextUrl><n.nextUrl_field.value/></nextUrl>
</n.do_login>
<n.set_var. name="error"><n.true/></n.set_var.>
</then>
</n.if.is_submitted_form>
<n.html>
<head>
<meta name="robots" content="noindex,nofollow"/>
<n.title.><t>Login</t></n.title.>
<n.email_field.focus/>
<script type="text/javascript">
var loginNextUrl = '<n.default. to="/"><n.nextUrl_field.value/></n.default.>';
</script>
</head>
<body>
<h1 class="weak-color"><t>Login</t></h1>
<img src="http://www.gravatar.com/avatar/d4a4d0f46d6480e58803dddb274c7579?s=100&r=pg&d=http%3A%2F%2Fn2.nabble.com%2Fimages%2Favatar100.png" alt="" />
<n.login_message/>
<n.if.var name="error">
<then.format_error
message="[t]Incorrect Login![/t]"
prompt="[t]Please re-enter your email/password and click Login.[/t]"
/>
</n.if.var>
<n.login_form/>
<n.register_now_section/>
</body>
</n.html>
</override_macro>