Task

Write a program that solves the odd word problem with the restrictions given below.



Description

You are promised an input stream consisting of English letters and punctuations.

It is guaranteed that:

 the words (sequence of consecutive letters) are delimited by one and only one punctuation,
 the stream will begin with a word,
 the words will be at least one letter long,   and 
 a full stop (a period, [.]) appears after, and only after, the last word.


Example

A stream with six words:

 what,is,the;meaning,of:life. 


The task is to reverse the letters in every other word while leaving punctuations intact, producing:

 what,si,the;gninaem,of:efil. 
while observing the following restrictions:

 Only I/O allowed is reading or writing one character at a time, which means: no reading in a string, no peeking ahead, no pushing characters back into the stream, and no storing characters in a global variable for later use;
 You are not to explicitly save characters in a collection data structure, such as arrays, strings, hash tables, etc, for later reversal;
 You are allowed to use recursions, closures, continuations, threads, co-routines, etc., even if their use implies the storage of multiple characters.


Test cases

Work on both the   "life"   example given above, and also the text:

 we,are;not,in,kansas;any,more.


let is_alpha c =
  c >= 'a' && c <= 'z' ||
  c >= 'A' && c <= 'Z'
 
let rec odd () =
  let c = input_char stdin in
  if is_alpha c
  then (let e = odd () in print_char c; e)
  else (c)
 
let rec even () =
  let c = input_char stdin in
  if is_alpha c
  then (print_char c; even ())
  else print_char c
 
let rev_odd_words () =
  while true do
    even ();
    print_char (odd ())
  done
 
let () =
  try rev_odd_words ()
  with End_of_file -> ()