Lena is working on a utility for ReviewThat Labs that generates a sequence of strings where each new entry describes the one before it. Each step involves looking at the previous string and recording the count and value of each group of identical digits. Mark reviewed her early draft and suggested making sure the tool could efficiently generate many versions of a sequence starting from just a single digit. For example, if the first entry is '1', the second becomes '11' to describe one 1, the third becomes '21' to describe two 1s, and the fourth results in '1211'.