tools.completion: Add utility word for searching for a string in a sequence of strings.
combinators.extras: Add twice, thrice words.
hash-sets: Support >alist on hash-sets or else tab-completion breaks. Can we write a unit test for this too?
ui.tools.listener.completion: fix use of search-vocab-names.
hash-sets: back out this change for now.
graphs: change closure to use a set and operate on sequences.