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From: Marius Amado Alves <amado.alves@netcabo.pt>
Newsgroups: comp.lang.ada
Subject: Re: Why can't you create a out of order subtype?
Date: Wed, 2 Feb 2005 20:35:24 +0000
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Date: 2005-02-02T20:35:24+00:00

>> declare
>>    M : Msg := Whatever;
>>    V : Vowelmsg;
>> begin
>>    V := Vowelmsg'Value (Msg'Image (M));
>>    -- if control reaches here then M is a vowel
>> exception
>>    when Constraint_Error => -- M is not a vowel
>> end;
>
> That's very, very, very slow...

No, it's only very, very slow :-)

If (and only if :-) that's too slow for the application then sure, use 
UC and 'Valid. And watch out for those uninitialized variables.