Missing information on Expressional Indexes

Folks,

I think this page should mention that the expressions used in an
expressional index need to be equivalent to an IMMUTABLE function, and
the docs don't say so. The change should also be back-patched.
<http://www.postgresql.org/docs/current/static/indexes-expressional.html>

I'm thinking that something along the lines of, "Expressions are
limited to those which are equivalent to an [link]IMMUTABLE[/link]
function."

Anybody have a better suggestion for wording?

Cheers,
David.
--
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