Earlier, I wrote about how to create a custom function in Google Sheets.  However, have you ever wanted to use that function in a completely different Google Sheet?  Turns out, it just does not magically work.  Instead, you must create a separate web app, then bring it into the desired Google Sheet through use of a library.  I found a number of resources, but the best step-by-step instructions were here:

http://webapps.stackexchange.com/questions/39604/how-to-re-use-google-apps-scripts-in-new-google-spreadsheet