Below is a cut and paste from the google apps script guide
Debug a library When you use the debugger in a project that includes a library you can step into a function of the included library. The code shows up in the debugger in view-only mode and at the right version.
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I can't make this work. I set a breakpoint in the bound script at the line that calls the function in the library script. I then click in "step into" and the bound script completes without any interaction from the library script. Any ideas?