Version declaration proposal

[Don]

I think I have a solution to the versioning problems.
My belief is that version statements are fine, and Walter's 'rats nest' 
argument is valid. Instead, I believe that version declarations limited 
to "version = XXX;" are too weak. They also have syntax which is subtly 
different to the rest of the language.
(version = A; version = B; looks as though you're changing the value of 
the 'version' variable).
My proposal is therefore:

--------
* Keep version statements unchanged.
* Change version declarations to make them almost identical to bool 
declarations:

version _versionidentifier_ = _versionexpression_;

where _versionexpression_ is a boolean expression involving only true, 
false, or other version identifiers, and standard boolean operators.
* If, in addition, we allow:

version _versionidentifier_ = extern;

for versions which are supplied from the command line, it can then 
become illegal to have a version statement referring to an undeclared 
version identifier.
(Any version identifier beginning with D_ is implicitly an "= extern;" 
identifier, so does not need to be declared).
Another possibile syntax is extern version _versionidentifier_; to make 
the syntax identical to normal declarations.

--------

This addresses a few problems:
* typos in version identifiers.
* Comprehensibility. When you see something like:
version (UseLargeSize) {
}

you have no idea if that version identifier declaration is buried in the 
code somewhere, or whether it is a command-line option. It could even be 
both!
* The effect of this would be to encourage you to create a block of 
version declarations at the start of the module, where they are easy to 
analyze and refactor.
* A lint tool could even enforce this, by disallowing any version 
declarations after the first normal declaration is encountered.

I believe this would reduce rats-nest versioning, and also eliminate the 
rats-nesting we have in version declarations right now.
Is there anything wrong with it?