let () = let s = read_line () in let n = String.length s in for i = 0 to n - 1 do let t = String.copy s in let d = (if i < n / 2 then i else n - i - 1) in for j = 0 to d - 1 do t.[j] <- ' '; t.[n - j - 1] <- ' ' done; t.[1] <- s.[1]; t.[n - 2] <- s.[n - 2]; print_endline t done

Note that non-ascii characters in the above source code will be escaped (such as \x9f).