Truncation (InstanceTopic, 3)

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Truncation is the process of cutting polytope facets to produce new polytopes. The kind of truncation can be specified by a Dx number.

Particular types of truncation include:

Truncates the polytope to its "midpoint". In odd dimensions, this is represented as the middle Coxeter-Dynkin node ringed. In even dimensions, the middle two nodes are ringed.
"Expands" the polytope. This is represented with the first and last nodes ringed. Etymology: "extra" is Latin for "outside" - you can imagine the ringed nodes as being on the "outside" (at the ends) of the diagram.