code: function attributes(attrs) {
    attrs = attrs || {};
    var ident = attrs.id || '';
    var classes = attrs.classes || [];
    var keyvals = [];
    Object.keys(attrs).forEach(function (k) {
      if (k !== 'classes' && k !== 'id') keyvals.push([k,attrs[k]]);
    });
    return [ident, classes, keyvals];
  }
docs: Returns an attribute list, constructed from the dictionary attrs.
  @param  {Object} attrs Attribute dictionary
  @return {Array}        Attribute list