# Append an element (`String`, other `Template`, etc.) at the end of the template.
	#
	# Should be either used externally to act on basic templates,
	# or internally in the `rendering` method of specific templates.
	#
	# Mixing the internal and external uses should be avoided because
	# the final behavior will depend on the lazy invocation of `rendering`.
	#
	#     var t = new Template
	#     t.add("1")
	#     t.add("2")
	#     assert t.write_to_string == "12"
	fun add(element: Writable) do
		assert not is_frozen
		content.add element
	end