Author: Urs Format: MarkdownItexfixed the formatting, making clearer what the entry means to be about,
added doi and author hyperlink
added `category: reference`-tag
<a href="https://ncatlab.org/nlab/revision/diff/Algebraic+Homotopy/5">diff</a>, <a href="https://ncatlab.org/nlab/revision/Algebraic+Homotopy/5">v5</a>, <a href="https://ncatlab.org/nlab/show/Algebraic+Homotopy">current</a>
fixed the formatting, making clearer what the entry means to be about,