midi tempo

[Noeck]

Hi,

I would like to specify a different tempo for the pdf and the midi
output. I’ve read
http://www.lilypond.org/doc/v2.18/Documentation/notation/displaying-rhythms.html#metronome-marks

But I found only how to specify it for both, or for one of them. What I
want is like:
\tempo "Allegro" 4 = 100
which should show up as »Allegro«
and the midi should be played as with 100 bpm.

I tried

\version "2.18.2"
{
  \tempo "Allegro"
  \once \set Score.tempoHideNote = ##t
  \tempo 4 = 100
  a
}

but I get:
  warning: Two simultaneous tempo-change events, junking this one

Cheers,
Joram