You can use symbols to define labels, or you can equate them to a
    specific value by a direct assignment statement. You can also use
    these symbols in expressions.

    Use the following rules to create user-defined symbols:

    o  Use alphanumeric characters, underscores (_),  dollar signs
       ($),  and periods (.). Any other character terminates the
       symbol.

    o  The first character of a symbol cannot be a number.

    o  The symbol must be no more than 31 characters long and must be
       unique.

    o  The symbol must not be a register name.

    o  The symbol cannot be one of the following conditional or macro
       directives:

       .ELSE          .ENDC          .ENDM
       .ENDR          .IF            .IF_FALSE (.IFF)
       .IF_TRUE       .IF_TRUE_      .IIF
       (.IFT)         FALSE
                      (.IFTF)
       .INCLUDE       .IRP           .IRPC
       .LIBRARY       .MACRO         .MCALL
       .MDELETE       .MEXIT         .NARG
       .NCHAR         .REPEAT

    In addition, by Digital convention:

    o  The dollar sign ($)  is reserved for names defined by Digital.
       This convention ensures that a user-defined name (that does
       not have a dollar sign) will not conflict with a Digital-
       defined name (that does have a dollar sign).

    o  Do not use the period (.)  in any global symbol name because
       many languages, such as Fortran, do not allow periods in
       symbol names.

    Macro names follow the same rules and conventions as user-defined
    symbols. User-defined symbols and macro names do not conflict;
    that is, you can use the same name for a user-defined symbol and
    a macro.