CSV::Parser::InputsScanner#keep_back

# File tmp/rubies/ruby-3.2.0/lib/csv/parser.rb, line 196
def keep_back
  # trace(__method__, :start)
  scanner, start, buffer = @keeps.pop
  if buffer
    # trace(__method__, :rescan, start, buffer)
    string = @scanner.string
    if scanner == @scanner
      keep = string.byteslice(start, string.bytesize - start)
    else
      keep = string
    end
    if keep and not keep.empty?
      @inputs.unshift(StringIO.new(keep))
      @last_scanner = false
    end
    @scanner = StringScanner.new(buffer)
  else
    if @scanner != scanner
      message = "scanners are different but no buffer: "
      message += "#{@scanner.inspect}(#{@scanner.object_id}): "
      message += "#{scanner.inspect}(#{scanner.object_id})"
      raise UnexpectedError, message
    end
    # trace(__method__, :repos, start, buffer)
    @scanner.pos = start
  end
  read_chunk if @scanner.eos?
end