We'll start with the `@echo` directive since it is very useful for debugging
and demonstrating future directives.
The `@echo` directive takes a single string or expression argument and prints it
to `stderr`.
```
@echo "Hello World"
```
Note that expressions are always printed in base 10.
```
@echo $1234 + $5678 ; Prints "26796"
```
Methods of constructing strings out of expressions and printing in other bases
will be covered later.