[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