year = raw input from the user
while year is not "bye":
    year = int(year)
    if before 1582:
        if divisible by 4:
            leap year
        else:
            not a leap year
    else:
        if divisible by 4:
            note: potential leap year
            if divisible by 100: exception
                note: potentially not a leap year
                if divisible by 400: exception
                    leap year
                else:
                    no exception, so it's not a leap year
            else:
                no exception, so: leap year
        else:
            not a leap year
    get new year from user via raw input