Author: Urs Format: MarkdownItexexpanded a bit and added more references
<a href="https://ncatlab.org/nlab/revision/diff/GSO+projection/3">diff</a>, <a href="https://ncatlab.org/nlab/revision/GSO+projection/3">v3</a>, <a href="https://ncatlab.org/nlab/show/GSO+projection">current</a>