Checking if a string is null

[Manfred Nowak]

Don Clugston wrote

> Take two arbitrary vectors with different directions, a and b.
> a*0 == b*0, even though the direction of a is completely different
> to that of b. This is the same model which is being used for
> arrays; 

This is not true for arrays:

Take two arbitrary arrays with different directions, i.e. different 
types, a and b: `(a ~ null) == (b ~ null)' is an error.

-manfred