def n_exec_stmt(self, node):
        """
        exec_stmt ::= expr exprlist DUP_TOP EXEC_STMT
        exec_stmt ::= expr exprlist EXEC_STMT
        """
        self.write(self.indent, 'exec ')
        self.preorder(node[0])
        if not node[1][0].isNone():
            sep = ' in '
            for subnode in node[1]:
                self.write(sep); sep = ", "
                self.preorder(subnode)
        self.println()
        self.prune()
exec_stmt ::= expr exprlist DUP_TOP EXEC_STMT
        exec_stmt ::= expr exprlist EXEC_STMT