The Exist function checks for the existence of the specified file.
Before processing actions on a file, it is a good practise to test first if the file exists. You will not get an error if in some circumstances, the file does not exist.
<filename>, string, name of the file to test. If no path is included in <filename>, the file is searched in the current working folder.
ret, integer. If <filename> exists, the return value is 1. If not, the return value is 0.
'Test if a program exists under Windows folder before launching it
If a = 1 then