Commits on Jun 5, 2018

1. appendix-IV: Refactor the license expression appendix …

   This is a large diff, but I aimed for restructuring/polishing without
   changing the end result (much).  I did make a few intentional changes:
   
   * Extended license-id to include appendix I.3 (deprecated licenses).
     We don't want folks using these in license expressions (because
     they're deprecated), but they are valid (or we would have removed
     them instead of just deprecating them).  That means that in some
     cases the nature of a string is unclear.  For example 'GPL-1.0+'
     could be the depreacted license-id, or it could be a
     simple-expression using the more-recently-deprecated GPL-1.0
     license-id and the + operator.  I don't think that's a problem
     though, because I can't think of a case where the ambiguity would
     matter.
   
   * I've allowed + for license-ref (it used to be only for license-id).
     There could be external licenses which offer a choice between
     only-this-version and or-later grants, and allowing + for
     license-ref makes it easier to support those licenses as they
     transition into the SPDX License List.  This isn't a big deal, but
     it avoids needing separate license-refs for the only-this-version
     and or-later grants if you need both.
   
   * I've added explicit whitespace handling, vs. the previous version
     which just discussed it in the text.  That way the ABNF is the sole
     source of normative syntax information.
   
   * I've added a paragraph addressing casing, based on discussion in
     [1].
   
   * I've added enclosed-license-expression, so consumers like the
     tag:value format can suggest/require it.  This allows for more
     precision in consumers (e.g. appendix V should be updated to require
     enclosed-license-expression), but I've left those other sections
     alone for this commit.  Ideally the tag:value line would be moved to
     a separate section that defined the tag-value format, but we don't
     have such a section yet [2].
   
   * I've added Gary's documentation for spdx:OrLaterOperator [3];
     previously there was no way to represent the + operator in RDF/XML.
   
   * I've added Gary's documentation for spdx:WithExceptionOperator [4].
     I think it's a bit odd that the XML operator represetation are using
     URLs instead of the SPDX IDs that the license expression syntax
     calls for.  That means you cannot convert between the two
     representations without an ID <-> URL map.  But we can address that
     later.
   
   * I've removed spdx:LicenseException, because we currently provide no
     other way for authors to define license exceptions.  We do define a
     way for them to define their own licenses [5], and currently authors
     have to use that to give a LicenseRef to a license+exception pair if
     their exception is not in our list.  Gary feels like we may return
     to this later (and I'd be happy giving users a way to define their
     own exceptions), but we're removing it for now [6].
   
   * I've fleshed out the documentation for the + operator to explain how
     it works with the AGPL-1.0.  Without this explaination, I think
     there's a risk that folks misinterpret ${ROOT}-${BASE_VERSION}+ as
     "allows ${ROOT}-${VERSION} for any ${VERSION} >= ${BASE_VERSION}",
     but that's not true.  Instead the proper interpretation is "allows
     ${ROOT}-${VERSION} and any other licenses allowed by ${ROOT-VERSION}
     which are based on 'any later version' grant".  For example, if the
     AGPL-2.0 had not been released, you could distribute
     AGPL-1.0-or-later code under the GPL-3.0-or-later, but *not* under
     the AGPL-3.0-or-later.
   
   The HTML comment avoids the ambiguous four-space indent after the
   list.  Without the comment, it could be parsed as a code block (which
   is what we want) [7] or a second paragraph of the final list entry [8]
   (which is not what we want).  The HTML comment closes the list to
   resolve the ambiguity.
   
   [1]: spdx#63
   [2]: spdx#22 (comment)
   [3]: spdx#37 (comment)
   [4]: spdx#37 (comment)
   [5]: https://github.com/spdx/spdx-spec/blob/cfa1b9d08903befdf03e669da6472707b7b60cb9/chapters/6-other-licensing-information-detected.md#6.1
   [6]: spdx#37 (comment)
   [7]: https://daringfireball.net/projects/markdown/syntax#precode
        "To produce a code block in Markdown, simply indent every line of
        the block by at least 4 spaces or 1 tab"
   [8]: https://daringfireball.net/projects/markdown/syntax#list
        "Each subsequent paragraph in a list item must be indented by
        either 4 spaces or one tab"

   

   wking committed Jun 5, 2018
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