Out for lunch

I have been reading this morning about an interesting set called the Cantor set, about which there is also a good article in wikipedia. But it bothers me that I do not recall reading about it before, despite doing a fair bit of mathematics when young. Was I out for lunch that day or has that chunk of memory just melted away?

Digging into the wikipedia article, I also read about compact sets, about which I do remember the odd snippet. But then there was an interesting concept called separation, something that comes in various varieties, all interesting and important, certainly in the glory days when topology was being invented - and I remember nothing about it. I comfort myself with the idea that it was all so basic that it was quickly buried in a mound of more serious matter, and so forgotten.