void setDocumentFieldName(String fieldName) {
        if (fieldName == null) {
            throw new IllegalArgumentException("Field name can not be null.");
        }
        this.fieldName = fieldName;
    }
Sets the name of the field whose value to retrieve.

@param fieldName The field name to set.
searchlib/src/main/java/com/yahoo/searchlib/expression/DocumentFieldNode.java