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Paul W. Abrahams writes - > > I see PIs as something you wish to pass on to a particular processor or class > of processors. My sense of the matter (admittedly, unsupported by any > documents I know of) is that they are, or ought to be, inessential to the > general interpretation of a document. In other words, a well-formed and > valid document ought to remain well-formed and valid if all PIs are omitted. > > Examples of PIs would be an indication that the following space character > should be non-breaking or that a page break would be desirable at this > point. These would be "tweaks" in the form of typographical hints, quite > possibly particular to a specific version of a document typeset with one > specific typesetting program, e.g., TeX. With these kinds of usages in mind, you could consider PIs to be a different kind of markup superimposed on a marked-up text. For example, an XML document that contained RTF would have two more-or-less orthogonal markup systems imposed on the underlying text. PIs are orthogonal to the rest of the XML markup. So in a way, PIs represent a little bit of some undefined orthogonal markup system. The XML provides the structural markup. Furthermore, the PIs do not produce a self-delineating structure, which again is quite different from (the rest of) XML. Mabe this is the basic reason why so many people are uncomfortable with PIs -they appear be be from a different, undefined system. Cheers, Tom Passin
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