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If you want to use this software you will need to compile it and then run the executable. 
For example in a GNU/Linux based operating system you will need to run the following commands:

To compile teh software
$ g++ orthodox_easter -o orthodox_easter 

Run the software
$ ./orthdox_easter 

This program is licenced under GPLv3 (http://www.gnu.org/licenses/gpl-3.0.html)