rle2(S) = RLE =>
  Ix = [1] ++ [I : I in 2..S.len, S[I] != S[I-1]] ++ [S.len+1],
  Diffs = diff(Ix),
  RLE = [Diffs[I].to_string() ++ S[Ix[I]].to_string() : I in 1..Diffs.len].join('').