rle3(S) = RLE =>
  rle3(S.tail(),S[1],1,[],RLE).

rle3([],LastChar,Count,RLE1,RLE) =>
  RLE = (RLE1 ++ [Count.to_string(),LastChar.to_string()]).join('').

rle3([C|T],LastChar,Count,RLE1,RLE) =>
  C == LastChar ->
    rle3(T,C,Count+1,RLE1,RLE)
  ;
    rle3(T,C,1,RLE1++[Count.to_string()++LastChar.to_string()],RLE).