@Override
    public DataType setOutputType(DataType output, TypeContext context) {
        super.setOutputType(output, context);
        var inputType = resolveBackwards(context);
        resolveForwards(context);
        return inputType;
    }
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