How to get rid of const / immutable poisoning (and I didn't even want to use them...)

[Enjoys Math via Digitalmars-d-learn]

I had to employ const / immutable to some things to get passed 
some compiler errors.  Then the poisoning continues.

How do I get this code to run?
String's will hold a T[] which actually will not be modified by 
the String methods ("immutable strings of T").

I did not want to use any immutable / const anywhere at first, 
but by passing in [1,2,3]