Add a handler to the pipeline used to initialize new documents.

        Args:
            handler (Handler) : a handler for this Application to use to
                process Documents

def add(self, handler):
        ''' Add a handler to the pipeline used to initialize new documents.

        Args:
            handler (Handler) : a handler for this Application to use to
                process Documents

        '''
        self._handlers.append(handler)

        # make sure there is at most one static path
        static_paths = set(h.static_path() for h in self.handlers)
        static_paths.discard(None)
        if len(static_paths) > 1:
            raise RuntimeError("More than one static path requested for app: %r" % list(static_paths))
        elif len(static_paths) == 1:
            self._static_path = static_paths.pop()
        else:
            self._static_path = None