Returns the statement object for the supplied identifier
        @type statement_id: string
        @param statement_id: statement identifier

def get_statement(self, statement_id):
        """
        Returns the statement object for the supplied identifier
        @type statement_id: string
        @param statement_id: statement identifier
        """
        if statement_id in self.idx:
            return Cstatement(self.idx[statement_id], self.type)
        else:
            return None