AddToPath:

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   Add a local directory to the path and return to the current directory.
   This function is designed to be executed from the parent function,
   i.e. the top of a module 
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   Form:
   AddToPath( dirName )
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   Inputs
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   dirName       (:)    The name of the directory to add. If omitted, the
                        current directory will be added.

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