I was reading the Proof to Theorem 3.54 in Rudin's Prininples of Mathematical Analysis. The theorem says the following:
Let $\sum a_n$ be a series of real numbers which converges but not absolutely. Suppose $-\infty\leq\alpha\leq\beta\leq\infty$. Then there exists a rearrangement $\sum a_n'$ with partial sums $s_n'$ such that $\liminf s_n'=\alpha$ and $\limsup s_n'=\beta$.
At some point in the proof, the author says "Let $P_1, P_2,\ldots$ denote the nonnegative terms of $\sum a_n$, in the order they occur"
I'm not sure what he means by this. Any ideas?