[sv-bc] Out-of-bound index for part select on concatenation

Hi,
In 1800-2009 draft LRM, a new construct is added as part select of 
concatenation. Is there any reason for which bit select is not allowed. 
For e.g.
{a, b}[2]

Also, LRM does not mention what will happen if out-of-bound index is 
used in the part select expression. For e.g.
{a, b}[-1:2]
Whereas, -1 is an invalid index here. Can anyone explain on that?

-- 
Regards
Surya




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