Type-safe access to the XML formats. See: http://0install.net/interface-spec.html http://0install.net/selections-spec.html
_ t types are just
Support.Qdom.element, but we add a
phantom type parameter to avoid mix-ups. e.g. a
[`command] t is an element
that we know is a <command>.
The type parameter is a polymorphic variant because many commands work on
multiple element types. For example, many elements have an 'interface'
attribute, so the
interface function accepts multiple types.
Generate a string identifying this element for use in error messages. Includes the source location, if known.