human
Solve the following python programming problem given the following code:
def __call__(self, environ, start_response) : 
    def replacement_start_response(status, headers, exc_info = None) : 
        '
            Overrides the default response if the status is defined in the
            Pecan app error map configuration.
            ' 
        try : 
            status_code = int (status.split (' ') [0]) 
except (ValueError, TypeError) : 
            raise Exception (('ErrorDocumentMiddleware received an invalid status %s' % status)) 
if (status_code in self.error_map) : 
            def factory(app) : 
                return StatusPersist (app, status, self.error_map [status_code]) 
raise ForwardRequestException (factory = factory) 
return start_response (status, headers, exc_info) 
app_iter = self.app (environ, <MASK>) 
    return app_iter 
    
    Given the code above, what is a proper replacement for <MASK>? Choose among: self, environ, start_response, app_iter
assistant
#Here is a proper replacement for <MASK>.
def replacement_start_response(