This is an example of a library. You may see a list of other libraries used on Rosetta Code at Category:Solutions by Library.
Wren-set is a module which adds set support to the Wren programming language. It consists of two classes: Set and Bag, the latter being a multi-set or set which allows objects to be repeated.
It is the thirteenth in a series of modules (listed on the language's [main page]) designed to assist with writing Rosetta Code tasks so the same code does not have to be written or copy/pasted time and time again thereby bloating a task's script code unnecessarily.
To use it you need to copy the source code (in the [talk page]) to a text file called set.wren and place this in the same directory as the importing script so the command line interpreter can find it.