The Program Heading and the Declaration Part  23  Example:  label  13,  00100,  99;  A  constant definition  introduces an identifier as  a synonym for a  constant. The symbol const heads the constant definition part, which  has the general form:  Figure 3.f  Syntax diagram for  ConstantDefinitionPart  where a constant is either a number, a constant identifier (possibly  signed), a character, or a string.  The use of constant identifiers generally makes a program more  readable and acts as a convenient documentation aid.  It  also allows you  to group machine- or example-dependent quantities at the beginning  of the program where they can be easily noted and changed or both.  This improves the portability and modularity of the program.  Figure 3.g  Syntax diagram for  Constant  Constantldentifier  UnsignedNumber  '----------+l  CharacterString  \--------------'  3.C.  Constant Definition Part 