How to convert a string containing two hex digits to a character?

[Leslie <jlturriff@centurytel.net>]

Hi, it's me again. :-)

        I need to be able to convert a string variable containing two
hexadecimal digits (e.g. "7C") to its equivalent character
value.  I see from ARM 3.5.5 "Operations of Discrete Types" that
I can use 

  function S'Val(Arg : universal_integer) return S'Base

where universal_integer would be the numeric value of the string,
but how do I get from "7C" to 16#7C# when 7C is a string value,
not a constant?  I.e.,

  Hex_Pair : unbounded_string;

  begin
    Hex_Pair := slice(some_string,1,2);
    Char_Code : character;
    -- now what?
  end;

Leslie