+ import std.math;
  import std.typecons;
- import std.math;
  /*
  Create a function which takes a string representing a file's name, and returns
-     'Yes' if the the file's name is valid, and returns 'No' otherwise.
? ----

+ 'Yes' if the the file's name is valid, and returns 'No' otherwise.
-     A file's name is considered to be valid if and only if all the following conditions 
? ----

+ A file's name is considered to be valid if and only if all the following conditions 
-     are met:
? ----

+ are met:
-     - There should not be more than three digits ('0'-'9') in the file's name.
? ----

+ - There should not be more than three digits ('0'-'9') in the file's name.
-     - The file's name contains exactly one dot '.'
? ----

+ - The file's name contains exactly one dot '.'
-     - The substring before the dot should not be empty, and it starts with a letter from 
? ----

+ - The substring before the dot should not be empty, and it starts with a letter from 
-     the latin alphapet ('a'-'z' and 'A'-'Z').
? ----

+ the latin alphapet ('a'-'z' and 'A'-'Z').
-     - The substring after the dot should be one of these: ['txt', 'exe', 'dll']
? ----

+ - The substring after the dot should be one of these: ['txt', 'exe', 'dll']
-     Examples:
? ----

+ Examples:
-     >>> file_name_check("example.txt")
? ----

+ >>> file_name_check("example.txt")
-     "Yes"
+ "Yes"
-     >>> file_name_check("1example.dll")
? ----

+ >>> file_name_check("1example.dll")
+ "No"
-     "No"
-     
  */
  string file_name_check(string file_name)