What is the legality of the following series? What is the legality of the following series? 1,11,21,1211,111221,312211,132221,11133211,31231221,13111213112211,111331121113212221...

Each terms enumerates the number of each present natural number in the previous one:

1

11 is one 1

21 is two 1

1211 is one 2 one 1 and so on