    feb :- this demonstrates a bug, say
           feb?
           and you get 29, but change to
           year = 2015
           and feb? crashes.
           There is no problem if we make feb a function of y;

    feb = 29, leap year
    feb = 28

    leap y = y % 400 == 0, y % 100 == 0
           = y % 4 == 0

    year = 2016
