For every enumeration subtype S, S'Enum_Val denotes a
function with the following spec:
function S'Enum_Val (Arg : Universal_Integer)
return S'Base;
The function returns the enumeration value whose representation matches the
argument, or raises Constraint_Error if no enumeration literal of the type
has the matching value.
This will be equal to value of the Val attribute in the
absence of an enumeration representation clause. This is a static
attribute (i.e. the result is static if the argument is static).