[Explain theory of Tcl's private interfaces.]

With 8.4 (?), Tk defines a namespace

    ::tk::unsupported

which ...

As "Migration to 8.4: Tk Private Commands" explains, there was a particular issue in the transition of Tk from 8.3 to 8.4. Details appear in the source distribution in

    library/unsupported.tcl

which installs as

    local/lib/tk$VERSION/unsupported.tcl