Thanks for this, Heiko.  Note there is no upper range check in the code here:

>> \reserveinserts is a macro of etex.sty:

| %% And we define \reserveinserts, so that you can say \reserveinserts{17}
| %% in order to reserve room for up to 17 additional insertion classes, that
| %% will not be taken away by \newcount, \newdimen, \newskip, or \newbox.
| 
| \outer\def \reserveinserts#1%
|  {\global\insc@unt\numexpr \et@xins \ifnum#1>\z@ -#1\fi \relax}
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