+ import std.math;
  import std.typecons;
- import std.math;
  /*
  You have to write a function which validates a given date string and
-     returns true if the date is valid otherwise false.
? ----

+ returns true if the date is valid otherwise false.
-     The date is valid if all of the following rules are satisfied:
? ----

+ The date is valid if all of the following rules are satisfied:
-     1. The date string is not empty.
? ----

+ 1. The date string is not empty.
-     2. The number of days is not less than 1 or higher than 31 days for months 1,3,5,7,8,10,12. And the number of days is not less than 1 or higher than 30 days for months 4,6,9,11. And, the number of days is not less than 1 or higher than 29 for the month 2.
? ----

+ 2. The number of days is not less than 1 or higher than 31 days for months 1,3,5,7,8,10,12. And the number of days is not less than 1 or higher than 30 days for months 4,6,9,11. And, the number of days is not less than 1 or higher than 29 for the month 2.
-     3. The months should not be less than 1 or higher than 12.
? ----

+ 3. The months should not be less than 1 or higher than 12.
-     4. The date should be in the format: mm-dd-yyyy
? ----

+ 4. The date should be in the format: mm-dd-yyyy
- 
-     >>> valid_date("03-11-2000")
? ----

+ >>> valid_date("03-11-2000")
+ true
-     true
- 
-     >>> valid_date("15-01-2012")
? ----

+ >>> valid_date("15-01-2012")
+ false
-     false
- 
-     >>> valid_date("04-0-2040")
? ----

+ >>> valid_date("04-0-2040")
+ false
-     false
- 
-     >>> valid_date("06-04-2020")
? ----

+ >>> valid_date("06-04-2020")
+ true
-     true
- 
-     >>> valid_date("06/04/2020")
? ----

+ >>> valid_date("06/04/2020")
+ false
-     false
-     
  */
  bool valid_date(string date)