{{ $username := .Get 0 }}
{{ $url := .Get 1 }}

{{ if not $url }}
  {{ errorf "mention shortcode requires both username and URL parameters. Usage: {{< mention \"username\" \"https://example.com/@username\" >}}" }}
{{ end }}

<a href="{{ $url }}" class="u-in-reply-to h-card">@{{ strings.TrimPrefix "@" $username }}</a>