Subtypes with Combined Ranges

[David Rasmussen <pinkfloydhomer@yahoo.com>]

An Ada-excercise reads as follows:

6.1 Write a program to count the number of occurrences of each letter of 
the alphabet typed as input at the keyboard. Using a subtype of 
Character as the index subtype of an array is a sensible way to do this.

My first thought was to do this:
subtype Alphabetical is Character range 'A'..'Z' | 'a'..'z';

But I get an error. What is the best type for this excercise?

/David