+ import std.math;
  import std.typecons;
- import std.math;
  /*
  Given a string representing a space separated lowercase letters, return an associative array
-     of the letter with the most repetition and containing the corresponding count.
? ----

+ of the letter with the most repetition and containing the corresponding count.
-     If several letters have the same occurrence, return all of them.
? ----

+ If several letters have the same occurrence, return all of them.
-     
-     Example:
? ----

+ Example:
-     >>> histogram("a b c")
? ----

+ >>> histogram("a b c")
-     ["a": 1L, "b": 1L, "c": 1L].nullable
? ----

+ ["a": 1L, "b": 1L, "c": 1L].nullable
-     >>> histogram("a b b a")
? ----

+ >>> histogram("a b b a")
-     ["a": 2L, "b": 2L].nullable
? ----

+ ["a": 2L, "b": 2L].nullable
-     >>> histogram("a b c a b")
? ----

+ >>> histogram("a b c a b")
-     ["a": 2L, "b": 2L].nullable
? ----

+ ["a": 2L, "b": 2L].nullable
-     >>> histogram("b b b b a")
? ----

+ >>> histogram("b b b b a")
-     ["b": 4L].nullable
? ----

+ ["b": 4L].nullable
-     >>> histogram("")
? ----

+ >>> histogram("")
-     ___null_dict___
? ----

+ ___null_dict___
- 
-     
  */
  Nullable!(long[string]) histogram(string test)