Hopefully someone can write a bit comparing and contrasting the
use of this variable and [TCLLIBPATH], describing to the developer
when each should and should not be used, and to the user of
Tcl applications when each are safe or not safe to use.`

Another useful thing to document here is what the format for
the value should be.  Is this a colon seperated series of
directories, a space seperated list of directories, or some
kind of list of paths to pkgindex.tcl files, or just a single
path?

See also [magic names].
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