@Override
    public DataType setInputType(DataType input, TypeContext context) {
        super.setInputType(input, context);
        var outputType = resolveForwards(context);
        resolveBackwards(context);
        return outputType;
    }
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