comparing characters in string

[Stribor40 <ikamzic@gmail.com>]

I am trying to do something with this function...For example I would pass 2 strings to function like this....

     function tt (a : in String ; b : in String) return Boolean is
         begin
              for i in a'Range loop
                  if a'(a'First) = b(b'First + 1) then
                     return false;
                  end if;
              end loop;
                     return true;
     end tt;
          
I am calling it like so...

b: Boolean := tt("123","321");

What the function is suppose to do is take first character from first parametar and compare it to every character in string b but not first character of string b.

Unfortunately my compiler doesnt like me and gives me this error...

subtype mark required in this context
found "a" declared at line 6
compilation error