[Rd] legitimate use of :::

[Gabor Grothendieck]

If ::: is disallowed then its likely that package developers will need
to export more functions to satisfy the consumers of those otherwise
hidden functions but if more functions are exported then there
will be a greater likelihood of conflicts among packages.

The problem seems to be that there are potentially three sorts of
functions here:

1. a function is hidden
2. a function is accessible via ::: but is not on the search path
3. a function is on the search path

The problem arises in attempting to force fit these three concepts
into only two
categories either by removing the first category (as was done previously)
or by removing the second category (which seems to be the new approach).