code: function getMapProperties(m, definitions, symbolizers, env) {
      var rules = {};
  
      _(m).each(function(value, key) {
          if (key in symbolizers) rules[key] = value;
      });
  
      definitions.filter(function(r) {
          return r.elements.join('') === 'Map';
      }).forEach(function(r) {
          for (var i = 0; i < r.rules.length; i++) {
              var key = r.rules[i].name;
              if (!(key in symbolizers)) {
                  util.error(env, {
                      message: 'Rule ' + key + ' not allowed for Map.',
                      index: r.rules[i].index
                  });
              }
              rules[key] = r.rules[i].ev(env).toObject(env)[key];
          }
      });
      return rules;
  }
docs: Find a rule like Map { background-color: #fff; },
  if any, and return a list of properties to be inserted
  into the <Map element of the resulting XML. Translates
  properties of the mml object at `m` directly into XML
  properties.
  
  @param {Object} m the mml object.
  @param {Array} definitions the output of toList.
  @param {Array} symbolizers map symbolizers
  @param {Object} env
  @return {String} rendered properties.