public FieldUpdate removeFieldUpdate(Field field) {
        return id2FieldUpdates.remove(field.getId());
    }
Adds all the field- and field path updates of the given document update to this. If the given update refers to a
different document or document type than this, this method throws an exception.

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