Non-static inner classes, do they pose an issue?

[neomatrix369]

After fixing issue #33, a discussion about non-static inner classes lead to the below inquiry, @Grundlefleck recommended investigating further:

In terms of inner classes, static inner classes are no issue at all. They do not have a reference to the enclosing class so state changed does not affect the outer class. Non-static inner classes may be an issue, and worth investigating more, but I don't think it should hold up this issue, as I suspect the builder pattern would be more common for immutable classes. Worth a look though, if you're interested.

If the outcome of the investigation helps prove that the initial theory was correct about non-static classes being a problem, it will be considered an issue for further investigation and fixing.