m-X completing reader -- counterintuitive behavior

[David Chapman <Zvona at OZ>]

In Symbolics 3670 Zwei in Release 6.0, IP-TCP 29.0, AISite 7.8,
microcode TMC5-IO4-MIC 319, FEP 22, on Lisp Machine Duane Allman:

I want m-X Find Files in Tag Table, so I type m-X f <sp> f <sp> i <cr>
and lose.  Lose because the second <sp> doesn't insert a space into the
buffer.  That happens, I suppose, because there is a Format File
command.  But I shouldn't get screwed by that; I shouldn't have to know
that Format File exists.  It's a violation of abstraction.  Presumably
this behavior is intended to pretty up the case where you use space at
the end of a command to complete, and since this is potentially the end
of a command...  But making it easy to figure out abbreviated command
sequences is a lot more important.