[sv-bc] Updated proposal for `keywords compatibility directive

I have attached a new update to the proposal to add a `keywords
compatibility directive. This proposal was approved in the last BC meeting
with some "friendly amendments". I had the action item to implement these
amendments in the final proposal text. One of those changes was to add an
example recommended by Shalom. I made the amendments, but have since
received feedback from Shalom that I did not capture his intent. The
version of the proposal attached adds in Shalom's feedback. The
added/changed examples are in blue to make them obvious.

I also received feedback that the wording in two paragraphs were ambiguous,
and the syntax BNF did not make it clear that quotes are required around the
versions specifier. I have corrected those ambiguities in the attached
proposal. The corrections are:

Move the quote marks in the BNF, to make it clear that the version specifier
must always be in quotes.

Clarify that there can be other versions specified beyond the ones listed in
the proposal. For example the Verilog-AMS standard can add version
specifiers for that standard. When we voted on this proposal, we discussed
specifically adding Verilog-AMS keywords to proposal, and it was decided
that this should be done by the Verilog-AMS standard, not the SystemVerilog
standard. I received feedback on the amended proposal, however, that it was
not clear as to whether other standards could add version specifiers to the
`keywords directive.

Clarify what should happen if a tool does not recognize the version
specifier given (e.g. there is a typo that specifies the version
"1364-19995"). Unrecognized version specifiers are an error.

The third ambiguity corrected in this proposal is to make it more clear that
implementations can specify alternate ways to establish a default set of
keywords for when there is no `keywords directive. This was already in the
proposal that was approved, but I received some comments that the wording
was too succinct.

The updates made in the attached proposal are in color, so that you can see
what changed. These changes are in keeping with the `keywords proposal as
it was approved, with the "friendly amendments" that were to be made.

This latest version of the proposal needs to be uploaded to the SV errata
data base.

Stu
~~~~~~~~~~~~~~~~~~~~~~~~~
Stuart Sutherland
stuart@sutherland-hdl.com
+1-503-692-0898