[variadic templates]n-th element of pack expansion syntax proposal

[Larry]

As mentioned in a post of comp.std.c++:

http://preview.tinyurl.com/yzg3dk5

there's a need for some way to get the nth element of a pack
expansion; however, the above post claims:

This is probably the most-requested feature for variadic templates,
and it never it made it because we never found a good, unambiguous
syntax.

Why wouldn't the syntax described by the following grammar production:

nth-expansion-element:
expansion-pattern '...[' constant-expression ']'

where:
expansion-pattern is the "pattern of the expansion" described
on p. 327 of:

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n3000.pdf

constant-expression is defined here:
http://www.csci.csusb.edu/dick/c++std/cd2/gram.html#gram.expr

be unambiguous? For example, boost::mpl::arg:

http://www.boost.org/doc/libs/1_40_0/libs/mpl/doc/refmanual/arg.html

could be implemented with the following code.

template< int n > struct arg
{
template< typename A...>
struct apply
{
typedef A...[n] type;
};
};

where:

A... is parsed as a expansion-pattern.
n is parsed as a constant-expression.
A...[n] is parsed as a nth-expansion-element.

Is there some document recording the various syntaxes tried and why
they were found ambiguous?

TIA.

-regards,
Larry

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