In parser programs it is often useful to examine the next character in
the input stream without removing it from the stream. This is called
“peeking ahead” at the input because your program gets a glimpse of
the input it will read next.
Using stream I/O, you can peek ahead at input by first reading it and
then unreading it (also called pushing it back on the stream).
Unreading a character makes it available to be input again from the stream,
by the next call to fgetc or other input function on that stream.