# Strip heading id
	fun strip_id(text: String): String do
		# strip id
		var b = new FlatBuffer
		for c in text do
			if c == ' ' then
				b.add '_'
			else
				if not c.is_letter and
				   not c.is_digit and
				   not allowed_id_chars.has(c) then continue
				b.add c
			end
		end
		var res = b.to_s
		if res.is_empty then res = "_"
		var key = res
		# check for multiple id definitions
		if headings.has_key(key) then
			var i = 1
			key = "{res}_{i}"
			while headings.has_key(key) do
				i += 1
				key = "{res}_{i}"
			end
		end
		return key
	end