code: function(name, url, params, options) {
          OpenLayers.Layer.Grid.prototype.initialize.apply(this, arguments);
  
          this.params = OpenLayers.Util.applyDefaults(
              this.params, this.DEFAULT_PARAMS
          );
  
          // unless explicitly set in options, if the layer is transparent, 
          // it will be an overlay
          if (options == null || options.isBaseLayer == null) {
              this.isBaseLayer = ((this.params.transparent != "true") && 
                                  (this.params.transparent != true));
          }
      }
docs: Constructor: OpenLayers.Layer.MapServer
  Create a new MapServer layer object
  
  Parameters:
  name - {String} A name for the layer
  url - {String} Base url for the MapServer CGI
  (e.g. http://www2.dmsolutions.ca/cgi-bin/mapserv)
  params - {Object} An object with key/value pairs representing the
  GetMap query string parameters and parameter values.
  options - {Object} Hashtable of extra options to tag onto the layer