code: function(value, node) {
          if (value instanceof OpenLayers.Filter.Function){
              this.writeNode("Function", value, node);
          } else {
              this.writeNode("Literal", value, node);
          }
          return node;
      }
docs: Method: writeOgcExpression
  Limited support for writing OGC expressions. Currently it supports
  (<OpenLayers.Filter.Function> || String || Number)
  
  Parameters:
  value - (<OpenLayers.Filter.Function> || String || Number)
  node - {DOMElement} A parent DOM element
  
  Returns:
  {DOMElement} Updated node element.