'infinity'::Interval should be added

At present we have a timestamp of 'infinity' and infinite ranges, but no
infinite interval

SELECT 'infinity'::timestamp;
works

SELECT 'infinity'::interval;
ERROR: invalid input syntax for type interval: "infinity"

Seems a strange anomaly that we should fix.

--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
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