public final void resolve(DocumentFieldValues fieldValues) {
        resolve((FieldTypes)fieldValues);
        fieldValues.getFullOutput();
    }
Sets the output type of this and returns the resulting input type, or null if it cannot be
uniquely determined.
Subtypes implement this by calling setOutputType(outputType, requiredType, VerificationContext context).
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/Expression.java