http://kor.i2p/books/The%20C%20Programming%20Language%20by%20K&R/chapter1.html
We prefer the symbolic constants IN and OUT to the literal values 1 and 0 because they make the program
more readable. In a program as tiny as this, it makes little difference, but
in larger programs, the increase in clarity is well worth the modest extra
effort to write it this way from the beginning.