Adds a iff clause to this statement

        :param clause: The clause that will be added to the iff statement
        :type clause: TransactionClause

def add_transaction_clause(self, clause):
        """
        Adds a iff clause to this statement

        :param clause: The clause that will be added to the iff statement
        :type clause: TransactionClause
        """
        if not isinstance(clause, TransactionClause):
            raise StatementException('only instances of AssignmentClause can be added to statements')
        clause.set_context_id(self.context_counter)
        self.context_counter += clause.get_context_size()
        self.transactions.append(clause)