5 · Check the unique-extension rule

A condensed group is solid when every assignment on probe points extends in exactly one way to all compatible integer measures.

Try this. Choose each example and read why it passes or fails. Pay attention to whether the problem is that an extension does not exist or that the group cannot contain all needed values.

Definition 5.1 gives the extension rule. Theorem 5.8 and Corollary 6.1 prove the facts used to classify these examples, so the activity reports consequences rather than proving them.