Using SML:

Challenge Problem: Write a function reasonable_date that takes a date and determines if it

describes a real date in the common era. A eal date" has a positive year (year 0 did not exist), a

month between 1 and 12, and a day appropriate for the month. Solutions should properly handle leap

years. Leap years are years that are either divisible by 400 or divisible by 4 but not divisible by 100.

(Do not worry about days possibly lost in the conversion to the Gregorian calendar in the Late 1500s.)