Well, using "abs()" for it would be a bad idea, because "|x|" (or "||x||") means "whatever is the distance I've defined between two elements of this particular

Well, using "abs()" for it would be a bad idea, because "|x|" (or "||x||") means "whatever is the distance I've defined between two elements of this particular space". For all that I know, the only reason to write "|x|" instead of "norm(x)" or "length(x)" is because brevity and because it is a well accepted notation. Edit: wrong post.