For basic_newline_filter (great idea; I hadn't even realized Macintosh had yet ANOTHER newline convention), it would be nice to be able to get the current line number and possibly even the character number on that line, for reporting errors in input. I realize this should be possible as a separate filter counting '\n' and non-'\n' characters, which handles the output of basic_newline_filter, but I don't think the cost of maintaining one or two counters would be too much a burden for those who don't need it (or if it is, provide versions with and without). Even nicer would be a call that extracts up to the previous N already consumed characters, for even more helpful context (I have already made a practice of showing the next N characters to be read in case of a fatal error, when the input is a file and not interactive). Obviously this could be a filter in its own right, and really has nothing to do with basic_newline_filter.