Any reason the removal/renaming of read-only registers should be permitted?

[G. Branden Robinson]

(This is for groff 1.24.)

I've opened the following ticket after getting a surprise.

https://savannah.gnu.org/bugs/?64131

In AT&T troff, you cannot remove a read-only register like `.l`.  It
throws no diagnostic; it silently refuses the request.

In groff, you can.  As far as I can tell, this has always been the case.

If you try to access these registers later, they will be re-created as
normal registers, with values of zero, and even if they don't screw up
the formatter internally, they will become unrecoverably useless for
documents and macro packages; there is no way to re-associate their
names with the internal formatter state they reflect by default.

Does anyone have any knowledge of whether this was a deliberate choice
in James Clark's design?  If so, what was the justification?

I intend to shut this door (and prohibit their renaming as well, an
inconceivable operation in AT&T troff).

Figure I'll probably lock up the `.T` string as well.

Regards,
Branden

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