  This is a preliminary draft of Chapter 7 of our forthcoming textbook
"Introduction to Cluster Algebras." Chapters 1-3 have been posted as
arXiv:1608.05735. Chapters 4-5 have been posted as arXiv:1707.07190. Chapter 6
has been posted as arXiv:2008.09189.
  This installment contains:
  Chapter 7. Plabic graphs
