The library should be defined as a Script Include. Referencing whatever is in the library can be done in two ways: naming the script include the same as the called function or constructor in that library, or by calling
gs.include('<Script Include name>');
after which whatever is in the Script Include is loaded and executed.
So if one has a library that is a function called (say) doSomething, one can create a Script Include called doSomething in-which the function with the same name is defined. Then one can just call that function from a widget's Server script. The system will auto-magically detect that the Script Include should be loaded and executed - I guess - just in time.
Or one can define the same function (doSomething) in a Script Include called (say) Library ABC. The one must first do the API call mentioned above and can use the defined function afterwards:
gs.include('Library ABC');
doSomething();
The limitation of the 2nd method is that only Script Include from the same scopes are loaded.
