add enum representation clauses

Author: leogermond

courses/fundamentals_of_ada/adv_280_low_level_programming.rst

@@ -208,6 +208,21 @@ Pack Aspect
       pragma Pack (Ar);
       -- Rec'Size is 36, Ar'Size is 1000
 
+-------------------------------
+Enum Representation Clauses
+-------------------------------
+
+* Can specify representation for each value
+* Representation must have increasing number
+
+.. code:: Ada
+
+   type E is (A, B, C);
+   for E use (A => 2, B => 4, C => 8);
+
+* Can use :ada:`E'Enum_Rep (A) = 2`
+* Can use :ada:`E'Enum_Val (2) = A`
+
 -------------------------------
 Record Representation Clauses
 -------------------------------