Error codes drafts

Hello,

There are now three different error codes drafts, representing four
different possible approaches to adding error codes to the spec.

1. PR #54, https://github.com/invisibleXML/ixml/pull/54
   Rendered at https://ndw.github.io/ixml/errcodes.html

   This draft inlines the error messages into the running prose:

      A mark is one of @, ^ or -, and indicates whether the item so
      marked will be serialised as an attribute (@), an element with its
      children (^), which is the default, or only its children (-). It
      is an error (S006) if an invalid character appears in the mark.

   Steven thought this style was too intrusive. He suggested using
   superscripts for the error codes instead.

2. PR #65, https://github.com/invisibleXML/ixml/pull/65
   Rendered at https://ndw.github.io/ixml/errcodes2.html

   This draft makes the error codes superscripts. In an effort to make
   the document accessible, the link comprises more than just the small
   superscript. CSS renders the link text a little more subtly than
   ordinary links.

      A mark is one of @, ^ or -, and indicates whether the item so
      marked will be serialised as an attribute (@), an element with its
      children (^), which is the default, or only its children (-). The
      mark must be a valid mark character⁰⁰⁶.

   (The “S” is missing from the superscript rendered in this message
   because there’s no superscript “S” in Unicode. Go figure.)

   (You can also see that in this particular case, I added a sentence to
   the prose so that there was a reasonable place to hang the error
   code. That wasn’t necessary for all of the errors in this draft, as
   it had been for proposal 1. Other editorial choices are possible.)

3. PR #74, https://github.com/invisibleXML/ixml/pull/74
   Rendered at https://ndw.github.io/ixml/errcodes3.html

   In response to a comment from Michael,
   https://github.com/invisibleXML/ixml/pull/65#issuecomment-1098290080
   this draft attempts a compromise between 1 and 2. Rather than
   placing the error codes in a superscript, it places them
   parenthetically.

      A mark must be one of @, ^ or -, (error S006) and indicates
      whether the item so marked will be serialised as an attribute (@),
      inserted as an element with its children (^), which is the
      default, or deleted, so that only its children are serialized (-).

   This can be viewed independently from the addition in proposal 4.

4. PR #74, https://github.com/invisibleXML/ixml/pull/74
   Rendered at https://ndw.github.io/ixml/errcodes3.html

   This proposal is an extension of proposal 3. In addition to placing
   the error codes parenthetically in the prose, it adds the full error
   text below the paragraph in which the error(s) occur. I’ve styled
   them with a pale red background so they stand out for review; we
   could choose a different presentation for the actual spec.

   This has the feature that it makes it easier to see the text of the
   error message when you’re reading the spec, and perhaps the drawback
   that the error messages “clutter up” prose of the spec.

I only did the first few errors in #74 and I didn’t update the appendix.
If we make a selection, I’m happy to construct a PR for the full
treatment.

                                        Be seeing you,
                                          norm

--
Norm Tovey-Walsh
Saxonica