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3.5.1 Enumeration Types

An enumeration_type_definition defines an enumeration type.
       enumeration_type_definition ::=
          (enumeration_literal_specification {, enumeration_literal_specification})
       enumeration_literal_specification ::=  defining_identifier | defining_character_literal
       defining_character_literal ::= character_literal
Legality Rules
The defining_identifiers and defining_character_literals listed in an enumeration_type_definition shall be distinct.
Static Semantics
Each enumeration_literal_specification is the explicit declaration of the corresponding enumeration literal: it declares a parameterless function, whose defining name is the defining_identifier or defining_character_literal, and whose result type is the enumeration type.
Each enumeration literal corresponds to a distinct value of the enumeration type, and to a distinct position number. The position number of the value of the first listed enumeration literal is zero; the position number of the value of each subsequent enumeration literal is one more than that of its predecessor in the list.
The predefined order relations between values of the enumeration type follow the order of corresponding position numbers.
If the same defining_identifier or defining_character_literal is specified in more than one enumeration_type_definition, the corresponding enumeration literals are said to be overloaded. At any place where an overloaded enumeration literal occurs in the text of a program, the type of the enumeration literal has to be determinable from the context (see 8.6).
Dynamic Semantics
The elaboration of an enumeration_type_definition creates the enumeration type and its first subtype, which is constrained to the base range of the type.
When called, the parameterless function associated with an enumeration literal returns the corresponding value of the enumeration type.

Examples of enumeration types and subtypes:
       type Day    is (Mon, Tue, Wed, Thu, Fri, Sat, Sun);
       type Suit   is (Clubs, Diamonds, Hearts, Spades);
       type Gender is (M, F);
       type Level  is (Low, Medium, Urgent);
       type Color  is (White, Red, Yellow, Green, Blue, Brown, Black);
       type Light  is (Red, Amber, Green); -- Red and Green are overloaded
       type Hexa   is ('A', 'B', 'C', 'D', 'E', 'F');
       type Mixed  is ('A', 'B', '*', B, None, '?', '%');
       subtype Weekday is Day   range Mon .. Fri;
       subtype Major   is Suit  range Hearts .. Spades;
       subtype Rainbow is Color range Red .. Blue;  --  the Color Red, not the Light

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