# Index a document
	#
	# With each new document, the `inverse_doc_frequency` must be updated.
	# By default, the method `update_index` is called after each call to
	# `index_document`.
	#
	# When processing batch documents, use `auto_update = false` to disable
	# the auto update of the index.
	fun index_document(doc: DOC, auto_update: nullable Bool) do
		for term, count in doc.terms_count do
			terms_doc_count.inc(term)
			if not inversed_index.has_key(term) then
				inversed_index[term] = new Array[DOC]
			end
			inversed_index[term].add doc
		end
		documents.add doc
		if auto_update == null or auto_update then update_index
	end