I disagree. It depends on where you draw the line and that is entirely subjective. For you, its when no textbooks are written, but for the purposes of this disc

I disagree. It depends on where you draw the line and that is entirely subjective. For you, its when no textbooks are written, but for the purposes of this discussion I think it would be more useful to talk about fields that are not so mainstream yet but that we have enough material to discuss. Its just a catch phrase, there is no real objective boundary