Array-like object indexing

[Ryan <rtarpine@hotmail.com>]

I've recently been playing with making all the standard data structures
in Ada to prepare myself for writing my first major project.  Currently 
I'm writing a resizable-array package.  I am trying to make it as 
similar to built-in arrays as possible, with functions for First, Length 
and other useful operations and attributes.  It is a generic package 
instantiated with a type for the elements to be stored and a type for 
indexing.  I've run into a small problem with respect to the indexing type.

I'm not sure how to represent the boundaries of an empty array without 
raising a constraint error in some cases.  If an array is being indexed 
by Natural, for example, and the first index should be 0 (as makes sense 
in many situations), then what should the First and Last functions 
(intended to work like their analogous attributes) return?  I cannot 
return 0 and -1 because -1 would raise a constraint error.

Should the responsibility for this rest on the package or the user? 
Would it be a better to return 1 and 0, or to force a user of the 
package to instantiate it with an indexing type that starts at -1 
instead even though -1 should never be used to actually look into the 
array?  What are your opinions on this?

Thank you,
Ryan Tarpine