Author: Urs Format: MarkdownItexAlso fixed the middle steps in the proof ([here](https://ncatlab.org/nlab/show/Poincaré+lemma#ProofOfHomotopyToChainHomotopy), which the page history shows I had messed up myself back in 2011).
<a href="https://ncatlab.org/nlab/revision/diff/Poincar%C3%A9+lemma/24">diff</a>, <a href="https://ncatlab.org/nlab/revision/Poincar%C3%A9+lemma/24">v24</a>, <a href="https://ncatlab.org/nlab/show/Poincar%C3%A9+lemma">current</a>
Also fixed the middle steps in the proof (here, which the page history shows I had messed up myself back in 2011).