 Parts of Sections 1.1–1.3, 2, 3.1–3.2, 5.2, and 6.2 are based on the  first version of this entry,  which was co-authored with Peter Bokulich. I thank John Manchak for producing figures 1, 2, and 3. I thank Jeremy Butterfield for supererogatorily detailed, thorough and elegant comments on a draft.