st: How to consecutively add elements to a macro

[Thomas Speidel <thomas@tmbx.com>]

***********EXAMPLE**************
local readyvar
use data1.dta
<do something>
save temp1
local readyvar birthday

use data2.dta
<do something>
save temp2
local readyvar <want to _add_ gender to the macro>

use data3.dta
<do something>
save temp3
local readyvar <want to _add_ smoking to the macro>

<...>
use temp1
merge id using temp2 temp3, sort
keep readyvar
**********END EXAMPLE************

My question is how to I add elements to an existing macro without 
having to re-type the whole content of the macro each time?  Also, if I 
understand correctly, local macros exist only if I run the whole do file 
at once.  In other words, if I run 'chunks' of the do files one at a 
time, I would not be able to use the macro that was created in the 
previous 'chunk'.  Is this one of those cases where a global macro is 
preferrable?

Thank you.
--
Thomas Speidel
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/