st: add index to the names of some variables

["Oleksandr Shepotylo" <shepotil@econ.umd.edu>]

Hi ,

I would like to add index i to some variable names. What is wrong with my
code:

variables: id a b c d

foreach var of varlist a-d {
  rename "`var'"  "`var'""_i"
}

Thank you

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