User blog comment:Vel!/Mega-Array Notation/@comment-24725252-20170809235639

This function doesn't terminate because there is no rule when b is equal to 1 and there is a third entry, and even if there was, it still will not terminate because all of the entires are increasing. Here is an exmple, {3,3,3}= {U(3),{3,2,3},U(3)} =  {U(3),{U(3),{3,1,3},U(3)},U(3)}  {U(3),{U(3),U(3),U(3)},U(3)}. Clearly, all three of the entries, increaed, which means the function will never halt.