# Fire the start of a mapping between `prefix` and `uri`.
	private fun fire_start_prefix_mapping(prefix: String, uri: String) do
		if not ns.declare_prefix(prefix, uri) then
			fire_error("The mapping between the prefix `{prefix}` and " +
					"the namespace IRI `{uri}` breaks a built-in " +
					"mapping. Ignoring the declaration.", null)
		end
		if content_handler != null then
			content_handler.start_prefix_mapping(prefix, uri)
		end
	end