# Look up the URI associated with a prefix in this context.
	#
	# Return `null` if no URI is associated with a specified prefix.
	#
	# Parameters:
	#
	# * `prefix`: prefix to look up.
	#
	# SEE: `NamespaceSupport.uri`
	fun uri(prefix: String): nullable String do
		if "" == prefix then
			return default_ns
		else if prefix_table == null then
			return null
		else
			return prefix_table.get_or_null(prefix)
		end
	end