[texhax] How to make the following condition: if... then... else...?

[H]

Hello, how are you.


How can I make the following condition in a .tex (main) document:  
if..., then..., else...?
For example, I have this:

%...
\input{pack.tex}
%...

But, the problem it never will available. So, I pretend use if...,  
then,... else... for no allowing LaTeX to make an error as follow as:

if "pack.tex" exists, then input (include) it, else ignore it.


I hope an answer as soon as you read this, please.

Thank you, very much.
Have a good day.

-HRE
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