content: gcdProg = runProgram $ callCC $ \exit ->          -- <--------+
    do                                              --          |
      start <- label                                -- <-----+  |
      output "Enter two integers, or zero to exit"  --       |  |
      nr <- readInt                                 --       |  |
      n <- get nr                                   --       |  |
      when (n == 0) $                               --       |  |
        do output "Exiting"                         --       |  |
           exit ()                                  -- ---------+
      mr <- readInt                                 --       |
      loop <- label                                 -- <--+  |
      n <- get nr                                   --    |  |
      m <- get mr                                   --    |  |
      when (n == m) $                               --    |  |
        do output ("GCD: " ++ show n)               --    |  |
           goto start                               -- ------+
      when (n > m) $ set nr (n - m)                 --    |
      when (m > n) $ set mr (m - n)                 --    |
      goto loop                                     -- ---+
  