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function Get_SSN (E : Employee_Type) return SSN_Type is
begin
   return E.SSN;
end;
function "<" (SSN : SSN_Type;
              E   : Employee_Type) return Boolean is
begin
   return SSN < E.SSN;
end;
function ">" (SSN : SSN_Type;
              E   : Employee_Type) return Boolean is
begin
   return SSN > E.SSN;
end;
package SSN_Keys is new Employee_Sets.Generic_Keys
  (Key_Type => SSN_Type,
   Key      => Get_SSN,
   "<"      => "<",
   ">"      => ">");