public void addAll(FieldUpdate update) {
        if (update == null) {
            return;
        }
        if (!field.equals(update.field)) {
            throw new IllegalArgumentException("Expected " + field + ", got " + update.field + ".");
        }
        addValueUpdates(update.valueUpdates);
    }
Adds all the {@link ValueUpdate}s of the given FieldUpdate to this. If the given FieldUpdate refers to a
different {@link Field} than this, this method throws an exception.

@param update The update whose content to add to this.
@throws IllegalArgumentException If the {@link Field} of the given FieldUpdate does not match this.
document/src/main/java/com/yahoo/document/update/FieldUpdate.java