types and subtypes

[ada_student@yahoo.com]

Given,

              subtype Positive is Integer range 1 .. Integer'Last;
              type String is (Positive range <>) of Character;

          the following should be illegal,

              subtype Count_To_Ten is Integer range 1 .. 10;
              subtype Ten_Characters is String (Count_to_Ten);

          and the following should be legal, instead

              subtype Count_To_Ten is Positive range 1 .. 10;
              subtype Ten_Characters is String (Count_to_Ten);

Why is Count_To_Ten allowed to be an Integer even though
the indices of String are declared as Positives?