The Herald-Stones of Gallowmere Pass

High in Gallowmere Pass stands a line of herald-stones, one after another along the old road. The first stone was carved with a string of digits. Each later stone was carved by the Pass Wardens reading the stone before it aloud, left to right, one run at a time. A run is a maximal block of consecutive identical digits. For each run they carve two things: how many digits the run contains, written in ordinary decimal, and then the digit itself.

A run's length is written in full, even when it is 10 or more. For example, a run of twelve 0s is carved as 120.

The Wardens' Lodge offers a bounty to whoever can say what is carved on any stone down the line without climbing the pass.

Write carveStone(seed, n):

• seed is a non-empty string of decimal digits (0-9), the carving on the first stone.
• n is a non-negative integer, the number of stones past the first.

Return the carving on the stone n steps after the first, as a string. Each step applies the reading rule above exactly once to the previous carving. When n is 0, return seed unchanged.