public void read(FieldBase field, StringFieldValue value) {
        try {
            if (isBase64EncodedElement(reader)) {
                throw new IllegalArgumentException("Attribute binaryencoding=base64 is not allowed for fields of type 'string'. To represent binary data, use type 'raw'.");
            } else {
                value.assign(reader.getElementText());
            }
        } catch (XMLStreamException | IllegalArgumentException e) {
            throw newException(field, e);
        }
    }
Optional test and set condition. Common for document/update/remove elements
This variable is either set in VespaXMLFieldReader#read (reader for document)
or in VespaXMLUpdateReader#read (reader for update).
document/src/main/java/com/yahoo/vespaxmlparser/VespaXMLFieldReader.java