@Override
    public StatementExpression convertChildren(ExpressionConverter converter) {
        return new StatementExpression(asList().stream()
                                               .map(converter::convert)
                                               .filter(Objects::nonNull)
                                               .toList());
    }
Returns the input fields which are (perhaps optionally) consumed by some expression in this statement.
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/StatementExpression.java