# Get option value as bool
	#
	# Return `true` if the value with that `key` is empty or equals `"true"`.
	# Return `false` if the value with that `key` equals `"false"`.
	# Return `null` in any other case.
	fun opt_bool(key: String): nullable Bool do
		if not has_key(key) then return null
		var value = self[key]
		if value.is_empty or value == "true" then return true
		if value == "false" then return false
		return null
	end