Checking if a string is null

[Manfred Nowak]

*** previous post superseeded ***

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 currently not true for arrays:

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

But it shouldn't be an error, because null arrays do not have a 
direction, i.e. some concrete type.

-manfred