I think I brought this up before, a long time ago, but what about
allowing a hash table as a storage class, for highly sparse arrays?
The only issue is that we have a number of flavors of hash tables, so
a little conditional code would be needed.  In addition, we can't have
just one hash table storage class: we need a hash table storage class
factory that takes a default object which is returned when the hash
table doesn't contain the demanded key.

Implementing the copier requires a walk through the source hash table
to find out which keys are between *from* and *to* and copy them to
the destination hash table.