While reading the paper "On Span Programs" by Karchmer and Wigderson (1993) I have come across the following definition:
[![enter image description here](https://i.stack.imgur.com/yGcpi.png)](https://i.stack.imgur.com/yGcpi.png)

The highlighted part of the definition contains the following elements that I find confusing:

1. What is the set represented by $[n]$? Is it the set of all of the natural numbers between 1 (or 0) and N?
2. What does $\epsilon = 0, 1$ mean? Does this mean that $\epsilon$ can be either 0 or 1? If so, shouldn't it be $\epsilon \in \{0,1\}$?