CHECK and IN (Sets in general) - What's the story?

Is there a known reason why the following definition fails
the parser at the IN keyword?:

create table kev_test ( my_tag char check (my_tag in ('A', 'B', 'C')));

The keyword IN is defined in Postgres' dialect (or it seems to be), so I'm
at a loss as to the problem.

K.
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Kevin O' Gorman - k_ogorman(at)kompass(dot)ie - programmer, Kompass Internet