( O = Output string   I = Input string   S = Subleq code )

  [ stack 0 ]                        is ip     (       --> s     )
  [ stack 0 ]                        is a      (       --> s     )
  [ stack 0 ]                        is b      (       --> s     )
  [ stack 0 ]                        is c      (       --> s     )

  [ over $ "" = iff 0
    else
      [ swap behead dip swap ]
    swap b share poke ]              is getch  ( O I S --> O I S )

  [ dup a share peek
    dip rot join unrot ]             is putch  ( O I S --> O I S )

  [ $ "" unrot
    0 ip replace
    [ dup ip share
      2dup     peek a replace
      2dup 1 + peek b replace
           2 + peek c replace
      3 ip tally
      a share -1 = iff getch again
      b share -1 = iff putch again
      dup  b share peek
      over a share peek -
      tuck dip [ b share poke ]
      1 <                    until
      c share dup ip replace
      0 <                    until ]
    2drop ]                          is subleq ( I S   --> O     )

  $ ""
  ' [  15  17  -1  17  -1  -1  16   1
       -1  16   3  -1  15  15   0   0
       -1  72 101 108 108 111  44  32
      119 111 114 108 100  33  13   0 ]
  subleq echo$