if (document.images)
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     nav1off = new Image(); nav1off.src="images/txt_quel_mediateur_off.gif";
     nav1on = new Image(); nav1on.src="images/txt_quel_mediateur_on.gif";
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     nav2on = new Image(); nav2on.src="images/txt_mediation_on.gif";
     nav3off = new Image(); nav3off.src="images/txt_comment_ca_marche_off.gif";
     nav3on = new Image(); nav3on.src="images/txt_comment_ca_marche_on.gif";
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  function rolloff(id)
  {
    if (document.images) {
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