# 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.