import std.math;
+ import std.typecons;
  /*
- 
-     Create a function that takes a string as input which contains only square brackets.
? ----

+ Create a function that takes a string as input which contains only square brackets.
-     The function should return true if and only if there is a valid subsequence of brackets 
? ----

+ The function should return true if and only if there is a valid subsequence of brackets 
-     where at least one bracket in the subsequence is nested.
? ----

+ where at least one bracket in the subsequence is nested.
- 
-     >>> is_nested("[[]]")
? ----

+ >>> is_nested("[[]]")
-     true
+ true
-     >>> is_nested("[]]]]]]][[[[[]")
? ----

+ >>> is_nested("[]]]]]]][[[[[]")
-     false
+ false
-     >>> is_nested("[][]")
? ----

+ >>> is_nested("[][]")
-     false
+ false
-     >>> is_nested("[]")
? ----

+ >>> is_nested("[]")
-     false
+ false
-     >>> is_nested("[[][]]")
? ----

+ >>> is_nested("[[][]]")
-     true
+ true
-     >>> is_nested("[[]][[")
? ----

+ >>> is_nested("[[]][[")
+ true
-     true
-     
  */
  bool is_nested(string string)