Cleared AA == and is have different results. Why?

[Brother Bill]

Page 119 of Programming in D

It seems odd that == null and is null have different values?
Is this a bug or feature?
If a feature, what is the meanings of == null vs. is null?

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;
     writeln("dayNumbers == null? ", dayNumbers == null);

     // Better, use 'is'
     writeln("dayNumbers is null? ", dayNumbers is null);
}

```

Console output:
```
dayNumbers == null? true
dayNumbers is null? false
```