isNumeric

Hi all,

Don't we need something like a function isNumeric to test for a numeric argument?
Seems to be handy, useful for instance for numeric aggregates, or no?

It seems COALESCE together with a cast would work as well, but
something like
  SUM(IF(isNumeric(?X), ?X, 0))
looks better - at least to me - than:
    SUM(COALESCE(xs:double(?X) , 0))

Opinions?

Axel