http://kor.i2p/books/The%20C%20Programming%20Language%20by%20K&R/chapter2.html
If not all values are specified, unspecified
values continue the progression from the last specified value, as the second
of these examples: enum escapes { BELL = '\a', BACKSPACE = '\b', TAB = '\t',
NEWLINE = '\n', VTAB = '\v', RETURN = '\r' };
enum months { JAN = 1, FEB, MAR, APR, MAY, JUN,
JUL, AUG, SEP, OCT, NOV, DEC };
/* FEB = 2, MAR = 3, etc. */ Names in different enumerations must be distinct.