code: function(to) {
          if ((to instanceof URI) === false) {
              to = new URI(to);
          }
  
          if (this.isAbsolute()                                                      &&
              to.isAbsolute()                                                        &&
              this.host === to.host                                                  &&
              this.port === to.port                                                  &&
              (this.scheme === to.scheme || (this.isFilePath() && to.isFilePath()) ) &&
              (this.path.indexOf(to.path) === 0 && (to.path.length === this.path.length ||
                                                    (to.path.length === 1 && to.path[0] === '/') ||
                                                    this.path[to.path.length] ===  '/'     )))
          {
              return (to.path.length === 1 && to.path[0] === '/') ? this.path.substring(to.path.length)
                                                                  : this.path.substring(to.path.length + 1);
          } else {
              return null;
          }
      }
docs: Get an URI relative to the given URI.
  @param  {String|zebkit.URI} to an URI to that the relative URI has to be detected.
  @return {String} a relative URI
  @method relative