private static void expectOperationSpecified(TensorModifyUpdate.Operation operation, String fieldName) {
        if (operation == null) {
            throw new IllegalArgumentException("Modify update for field '" + fieldName + "' does not contain an operation");
        }
    }
Reader of a "modify" update of a tensor field.
document/src/main/java/com/yahoo/document/json/readers/TensorModifyUpdateReader.java