st: sub-dataset by variable numbers

["Jian'an Luan" <jal42@medschl.cam.ac.uk>]

Dear Statalist

I wonder if I can select a sub-dataset by variables without given
variable names but using variable numbers, something like the command
when select a sub-dataset by observations ".keep if _n>1001 & _n<2000".
 
Any help will be appreciated.
 
Jianan

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