Why is TAMIL SIGN VIRAMA (pulli) not Alphabetic?

[Doug Ewell via Unicode]

SundaraRaman R wrote:

> but the very common pulli (VIRAMA)
> is neither in Lo nor has 'Other_Alphabetic', and so leads to
> concluding any string containing it to be non-alphabetic.

Is this definition part of Unicode? I thought the use of General 
Category to answer questions like "this sequence is a word" or "this 
string is alphabetic" was much more complex than that. (I'm not even 
sure what the latter means, for any script with any sort of combining 
mark.)

Richard Wordingham wrote:

> The effects of virama that spring to mind are:
>
> (a) Causing one or both letters on either side to change or combine to
> indicate combination;
>
> (b) Appearing as a mark only if it does not affect one of the letters
> on either side;
>
> (c) Causing a left matra to appear on the left of the sequence of
> consonants joined by a sequence of non-visible viramas.

Most of these don't apply to Tamil, of course.

--
Doug Ewell | Thornton, CO, US | ewellic.org