Associated Arrays and null. dict == null vs. dict is null

[Brother Bill]

What does dict == null mean vs dict is null, when dict is a 
dictionary?

Is this table accurate?
```
  dict == null    dict is null    # elements   memory allocated
       true           true            0             no
       true           false           0             yes
       false          false           1+            yes
       false          true     (Is this a possibility, and if so, 
how to create it?)
```

source/app.d
```
import std.stdio;

void main()
{
     // value[key]
     int[string] dayNumbers =
          //   key     : value
         [
             "Monday": 0, "Tuesday": 1, "Wednesday": 2, 
"Thursday": 3, "Friday": 4,
             "Saturday": 5, "Sunday": 6
         ];

     dayNumbers.remove("Tuesday");
     // writeln(dayNumbers["Tuesday"]);		// ← run-time ERROR

     dayNumbers.clear;                       // Remove all elements

     // Best to check length to see if associative array is empty, 
rather than comparing to null.
     // Checking to see if an associative array is null only tells 
you if it has been allocated memory.

     // dayNumbers has no elements
     writeln("dayNumbers == null? ", dayNumbers == null);    // 
true

     // dayNumbers has allocated memory, although no elements
     writeln("dayNumbers is null? ", dayNumbers is null);    // 
false

     writeln("dayNumbers.length: ", dayNumbers.length); // 0
     writeln;

     int[string] aNonInitializedDictionary;
     writeln("aNonInitializedDictionary is null? ", 
aNonInitializedDictionary is null); // true
     writeln("aNonInitializedDictionary == null? ", 
aNonInitializedDictionary == null); // true
     writeln("aNonInitializedDictionary.length: ", 
aNonInitializedDictionary.length); // 0
     writeln;

     int[string] aPopulatedDictionary;
     aPopulatedDictionary["Alpha"] = 1;
     writeln("aPopulatedDictionary is null? ", 
aPopulatedDictionary is null); // false
     writeln("aPopulatedDictionary == null? ", 
aPopulatedDictionary == null); // false
     writeln("aPopulatedDictionary.length: ", 
aPopulatedDictionary.length); // 0
}
```

Console output
```
dayNumbers == null? true
dayNumbers is null? false
dayNumbers.length: 0

aNonInitializedDictionary is null? true
aNonInitializedDictionary == null? true
aNonInitializedDictionary.length: 0

aPopulatedDictionary is null? false
aPopulatedDictionary == null? false
aPopulatedDictionary.length: 1
```