code: function(id) {
      // if the style doesn't exist return null
      var style = this.model.styles[id];
      if (!style) return null;
  
      // have we built this model before?
      var model = this.index.model[id];
      if (model) return model;
  
      // build a new model
      model = this.index.model[id] = {};
  
      // -------------------------------------------------------
      // number format
      if (style.numFmtId) {
        var numFmt = this.index.numFmt[style.numFmtId] || NumFmtXform.getDefaultFmtCode(style.numFmtId);
        if (numFmt) {
          model.numFmt = numFmt;
        }
      }
  
      function addStyle(name, group, styleId) {
        if (styleId) {
          var part = group[styleId];
          if (part) {
            model[name] = part;
          }
        }
      }
  
      addStyle('font', this.model.fonts, style.fontId);
      addStyle('border', this.model.borders, style.borderId);
      addStyle('fill', this.model.fills, style.fillId);
  
      // -------------------------------------------------------
      // alignment
      if (style.alignment) {
        model.alignment = style.alignment;
      }
  
      return model;
    }
docs: given a styleId (i.e. s="n"), get the cell's style model objects are shared where possible.