changing tags-file-name

["James K. Lowden" <jklowden@speakeasy.net>]

I decided to broaden a tags search by scanning from a higher point in
the source tree.  So I want to tell xref to use a different file.  But
set-variable says there's no such variable: 


The name of the file AIUI is tags-file-name, which C-h v describes as: 

tags-file-name is a variable defined in ?etags.el?.
Its value is
"/home/jklowden/projects/3rd/libdwarf/code/dwarfgen/TAGS"

  This variable is safe as a file local variable if its value
  satisfies the predicate ?stringp?.

Documentation:
File name of tags table.
To switch to a new tags table, setting this variable is sufficient.
[...]

When I use M-x set-variable, it rejects tags-file-name.  For
"tags-" completion it says, 

Possible completions are:
tags-add-tables 	tags-apropos-additional-actions
tags-apropos-verbose 	tags-case-fold-search
tags-compression-info-list 	tags-loop-revert-buffers
tags-revert-without-query 	tags-table-list
tags-tag-face

Why would emacs display a variable name, document that setting is
fine, and not be able to set it?  

Is there an easier way to change the tags file name (or add one and
drop one) that I'm overlooking? 

Thanks.  

--jkl


--jkl