Checks whether or not the elements of its input are leap years.
numeric vector of years
y is a leap year if it is evenly divisible by 4
and either it is not evenly divisible by 100 or it is evenly
divisible by 400, i.e.,
y%%4 == 0 & (y%%100 != 0 | y%%400 == 0).
logical vector of same length as
y indicating whether or not
the given years are leap years.