metronome mark

[Libero Mureddu]

Hi!
Can you help me to understand why the metronome marking doesn't appear 
in the following example?
I'm quite sure that I'm missing something very simple, but I can't 
solve this.
I'm putting \tempo everywhere with no results.

Libero Mureddu

P.S. Thanks to Mats for the staff size solution!

Ibook g3 800
mac os 10.3.4
lilypond 2.2.5

% ----- Piano -----
mdSuperiore = \notes {
  \key g \major
  \tempo 4=120
 % \voiceOne
 \mark \default
 r4 r8 \relative g'' {g16 fis e d c b a}
  \relative g' {g fis e }\time 2/4
}

mdInferiore = \notes {
 % \voiceTwo

}

msSuperiore = \notes {
  % \voiceOne
  }

msInferiore = \notes {
%  \voiceTwo
%  \key g \major
%  \relative c {
%    \repeat "tremolo" 16 { g32 d'32 } }
  }


PianoMD = {
  \clef treble
  \context Staff <<
    \context Voice = one \mdSuperiore
    \context Voice = two \mdInferiore
  >>
}

PianoMS = {
  \clef bass
  \context Staff <<
  \context Voice = One \msSuperiore
  \context Voice = Two \msInferiore
>>
}

piano = {
  \context PianoStaff <<
    \override PianoStaff.VerticalAlignment #'forced-distance = #8
    \override PianoStaff.TimeSignature  #'style = #'numbered
    \set PianoStaff.instrument = "Pianoforte       "
  \context Staff = upper \with {
    fontSize = #-2
    \override StaffSymbol #'staff-space = #(magstep -2)
  }
  \PianoMD
  \context Staff = lower \with {
    fontSize = #-2
    \override StaffSymbol #'staff-space = #(magstep -2)
  }
  \PianoMS
>>
}
% ----- Contrabbasso -----

basso = \notes {
    \tempo 4=120
  \key g\major
  | \relative c {g8. g'8 g,8 g'8. [g,8 g'] g,8~ [
  | g16 g'8 g,] g'8.}
 }

basso = {
  \set Staff.instrument = "Contrabbasso"
  \override Staff.TimeSignature  #'style = #'numbered
  \clef bass
  \context Staff <<
    \basso
  >>
}

% ----- partitura -----
\score {
  <<
  \context PianoStaff = piano \piano
  \context Staff = basso \basso
>>
\paper {
  \context {
    \RemoveEmptyStaffContext    
  }
}
}

Libero Mureddu
Vanha Viertotie, 21 As. 415
00350 Helsinki - Finlandia
Tel. +358-9-5808415
Mob. +358-41-7702668
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