Creating an index on a repeated nested field

Hi! I have the following record definition:

message TopLevel {
  repeated Nested nested = 1;
message Nested {
  optional string val = 1;

How do I create an concat index on TopLevel’s nested.val?

I.e., if I had a record {nested: [{val: 1}, {val: 2}]}, I want an index that has a single entry, [1, 2].

I believe this should be an index defined as:

Key.Expressions.field("nested", FanType.Concatenate).nest("val")