  We generalise the notion of cluster structures from the work of
Buan-Iyama-Reiten-Scott to include situations where the endomorphism rings of
the clusters may have loops. We show that in a Hom-finite 2-Calabi-Yau
category, the set of maximal rigid objects satisfies these axioms whenever
there are no 2-cycles in the quivers of their endomorphism rings.
  We apply this result to the cluster category of a tube, and show that this
category forms a good model for the combinatorics of a type B cluster algebra.
