st: conditional "keep" statement

[David Souther <davidsoutheremail@gmail.com>]

I'm combining (append & merge) hundreds of files which contain
hundreds or thousands of variables each, and at each stage of the
merge/append, I'd like to variables only if they are in a list that I
specify.
The issue is that not all of these datasets contain all the variables
in this list.  So, if I create a macro of my list,

local myvars  a1 a2 a3 d1 d2 d3

and then specify,

keep `myvars'

in the loop for each combination of data, I get the error that some
variable doesn't exist when it's not in the dataset.  I tried getting
around this with the -capture- command,

cap keep `myvars'

and using -ds-

ds `myvars'
keep `r(varlist)'

or,

ds `myvars', not
drop `r(varlist)'

and -ds- in combination with -capture- to no avail.   Any other ideas
(besides specifying the hundreds of variables to keep for each/every
loop)??

D.S.
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