# File lib/rmail/parser/pushbackreader.rb, line 85
      def standard_read_chunk(size)
        unless size.is_a?(Fixnum) && size > 0
          raise ArgumentError,
            "Read size (#{size.inspect}) must be greater than 0."
        end
        if @pushback
          chunk = @pushback
          @pushback = nil
        elsif ! @input.nil?
          chunk = @input.read(size)
        end
        return chunk
      end