**Descriptor Sequence.**

The descriptor sequence is a sequence of numbers in which the digits of each number describe the preceding number. The first number is 1. This number consists of one 1, so the second number is 11 (that is, one-one). This consists of two 1's, so the third term is 21. This consists of one 2 and one 1, so the fourth term is 1211. The first six numbers in the sequence are:

1, 11, 21, 1211, 111221, 312211.

Prove that the string 13113 never occurs.