Backwards incompatible change, but makes
it easier to reason about non-idiomatic searches:
now f specifies what is sought.
R=gri
CC=golang-dev
https://golang.org/cl/3195042
Change comment to be more generic,
with indexed data structure search as
one common use case.
Fix typo []data.
R=gri, rog
CC=golang-dev
https://golang.org/cl/3159041