link datetime,printf

procedure main(A)  # five weekends
   printf(  "There are %d months from %d to %d with five full weekends.\n",
            *(L := fiveweekends(s := 1900, f := 2100)), s,f)
   printf("The first and last five such months are:\n")
   every printf("%s\n",L[1 to 5]|"..."|L[-4 to 0])
   printf(  "There are %d years without such months as follows:\n",
            *(M := Bonus(s,f,L)))
   every printf("%s\n",!M)
end

procedure fiveweekends(start,finish)
   L := []        # months years with five weekends FRI-SUN
   every year := start to finish & month := 1 to 12 do
      if month = (2|4|6|9|11) then next
      else if julian(month,1,year) % 7 = 4 then
         put(L,sprintf("%d-%d-1",year,month))
   return L
end

procedure Bonus(start,finish,fwe)
every insert(Y := set(), start to finish)
every insert(F := set(), integer(!fwe ? tab(find("-"))))
return sort(Y--F)
end