go ?=>
  println("Months with five weekends:"),
  FiveWeekends = [ [Year,Month] : Year in 1900..2100, Month in [1,3,5,7,8,10,12], dow(Year,Month,1) == 5],
  WLen = FiveWeekends.len,
  println(take(FiveWeekends,5)),
  println("..."),
  println(drop(FiveWeekends,WLen-5)),
  println(len=WLen),
  nl,

  println("Years w/o five weekends:"),
  FiveWeekendYears = [Year : [Year,_] in FiveWeekends].remove_dups,
  NoHitYears = [Year : Year in 1900..2100, not member(Year,FiveWeekendYears)],
  NHLen = NoHitYears.len,
  println(take(NoHitYears,5)),
  println("..."),
  println(drop(NoHitYears,NHLen-5)),
  println(len=NHLen),
  nl.
go => true.

% Day of week, Sakamoto's method
dow(Y, M, D) = R =>
  T = [0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4],
  if M < 3 then
     Y := Y - 1
  end,
  R = (Y + Y // 4 - Y // 100 + Y // 400 + T[M] + D) mod 7.

% Days in a month.
max_days_in_month(Year,Month) = Days =>
  if member(Month, [1,3,5,7,8,10,12]) then
    Days = 31
  elseif member(Month,[4,6,9,11]) then
    Days = 30
  else
    if leap_year(Year) then
     Days = 29
   else
     Days = 28
   end
  end.

leap_year(Year) =>
  (Year mod 4 == 0, Year mod 100 != 0)
  ;
  Year mod 400 == 0.