Darren,
IMHO, we should not add context-specific tags unless they are absolutely
required. As you correctly pointed out, the addition of context-specific
tags will interfere with backwards compatibility in some cases.
You stated:
That way we would not have to rely of the definition of BuiltInContentType,
ExternalContentType and ExternalContentWithSubtype being unique from a tag
perspective, and if they ever changed we would not have to worry about
amending this definition retrospectively.
I don't believe that it causes any extra work to add a context-specific tag
once it has become absolutely necessary. For example, if one or more of the
BuiltInContentType, ExternalContentType or ExternalContentWithSubtype
definitions syntaxes are changed such that they are no longer unique, then
the implementor can change her ASN.1 software to implement the new
content-specific tag in conjunction with implementing the changed syntax.
In other words, the ASN.1 software must be changed anyway, so adding support
for the context-specific tag is a trivial amount of work.
- John