public static Dimension indexed(String name) {
            return new IndexedUnboundDimension(name);
        }
Returns the dimension resulting from combining two dimensions having the same name but possibly different
types:

[N] + [M] = [ min(N, M) ]
[N] + [] = []
[] + {} = {}
vespajlib/src/main/java/com/yahoo/tensor/TensorType.java