private String valueToString(ToStringContext<NAMETYPE> context) {
            if (label != null) {
                return TensorAddress.labelToString(label);
            } else {
                return index.toString(context);
            }
        }
Returns the index expression for this dimension, or empty if it is not a number
vespajlib/src/main/java/com/yahoo/tensor/functions/Slice.java