[Rd] support of `substitute(...())`

[Dénes Tóth]

Dear R Core Team,

I learnt approx. two years ago in this mailing list that one can use the 
following "trick" to get a (dotted pair)list of the ellipsis arguments 
inside a function:

`substitute(...())`

Now my problem is that I can not find any occurrence of this call within 
the R source - the most frequent solution there is 
`substitute(list(...))[-1L] `

I would like to know if:
1) substitute(...()) is a trick or a feature in the language;
2) it will be supported in the future;
3) when (in which R version) it was introduced.

A hint on where to look for the machinery in the R source would be also 
appreciated.

Regards,
Denes