[tex-eplain] \@getoptionalarg bug ? (or, more likely, ignorance)

[John Frampton]

I think the code

\makeatletter
X\@getoptionalarg\relax[dummy] Y
\resetatcatcode

should not introduce a space between X and Y.  But it does.  Am I 
mistaken about how \@getoptionalarg is supposed to work?

Thanks for any help with this.