The index could allow to identify what's the instance that is doing the circular reference (maybe in the future could be added support to set a reference to it when decoding the object, maybe exporting someway the list of seen objects).
The limit to 80 columns makes it easier to read the code, I didn't do it on the regular expresion but it would be also possible setting it by a string and a RegEx object constructor.
The index could allow to identify what's the instance that is doing the circular reference (maybe in the future could be added support to set a reference to it when decoding the object, maybe exporting someway the list of seen objects).
The limit to 80 columns makes it easier to read the code, I didn't do it on the regular expresion but it would be also possible setting it by a string and a RegEx object constructor.