st: Check whether ado file is installed

[Jan Sauermann <jan.sauermann@gmail.com>]

Dear Stata list,

I work on different computers and would Stata to automatically check
whether a number of ado-files is installed on a local machine. My idea
was to write something like

[a command which checks whether one ado-file is installed / or not]
if package!=installed {
    ssc install package
}

My problem is basically to find a command which (1) checks whether an
ado file is installed and (2)  gives me a local as return.

Best regards,

Jan
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