Author: Tim_Porter Format: MarkdownItexFixed some typos and copied references to the end of the entry.
<a href="https://ncatlab.org/nlab/revision/diff/crossed+square/18">diff</a>, <a href="https://ncatlab.org/nlab/revision/crossed+square/18">v18</a>, <a href="https://ncatlab.org/nlab/show/crossed+square">current</a>
Fixed some typos and copied references to the end of the entry.