% month(Name, Days, Next) when Name has Days days and is followed by Next.

month(jan, 31, feb).  
month(feb, 29, mar):-leapyear,!.
month(feb, 28, mar).
month(mar, 31, apr).  month(apr, 30, may).
month(may, 31, jun).  month(jun, 30, jul).  month(jul, 31, aug).
month(aug, 31, sep).  month(sep, 30, oct).  month(oct, 31, nov).
month(nov, 30, dec).  month(dec, 31, jan).

% month(X,_,_) will list months in order
% month(X,D,_) will give the days for month X
% month(X,_,N) will give the next month after X

% --------------or---------------

% month_days(Name, Days) and  when Name has Days days.
% month_next(Name, Next) and when Name is followed by Next.

month_days(feb, 29):-leapyear,!.
month_days(feb, 28).
month_days(M, 31):-member(M, [jan, mar, may, jul, aug, oct, dec]).
month_days(M, 30):-member(M, [apr,jun,sep,nov ]).

month_next(jan, feb).  month_next(feb, mar).
month_next(mar, apr).  month_next(apr, may).
month_next(may, jun).  month_next(jun, jul).  month_next(jul, aug).
month_next(aug, sep).  month_next(sep, oct).  month_next(oct, nov).
month_next(nov, dec).  month_next(dec, jan).

% month_next(X,_) will list months in order
% month_days(X,D) will give the days for month X
% month_next(X,N) will give the next month after X