Reference to an object disregarding mutation - what's the trick?

[BCS]

Reply to Manfred,

> Steve Teale wrote
> 
>> OK, here's the full story.
>> 
> What clumsy data structure might be organized by that?
> 
> That seems to be
> arr= arr[ 0 .. findex] ~ newnode ~ arr[ findex .. arr.length];
> and nothing more.
> -manfred
> 

yes that is what it seems to be, however, it skips the guarantied reallocation 
of the cat version