Grepping through my text copy. :) I find the following:
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xx These recondite directives exist, but not for public use:
Default Dictionary Listsymbols Listdict Listverbs Lowstring Stub
System_file Trace Btrace Etrace Ltrace Notrace Nobtrace Noetrace
Noltrace
(all in the TM). The one low-level directive which might be of practical use
is Abbreviate (see '28), an economy measure for enormous games.
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It seems that Stub is a pretty cryptic corner of Inform. It will be
documented in the Technical manual when it comes out (that's what "TM" is
for). If there is a Tech manual still at GMD, you might want to have a
look at it, even though it's an older version.
/Steve