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 9.35.5 String Substitution
 --------------------------
 
 A subset of allowable symbols (which we'll call subsyms) may be assigned
 arbitrary string values.  This is roughly equivalent to C preprocessor
 #define macros.  When `as' encounters one of these symbols, the symbol
 is replaced in the input stream by its string value.  Subsym names
 *must* begin with a letter.
 
    Subsyms may be defined using the `.asg' and `.eval' directives
 ( `.asg' TIC54X-Directives,  `.eval' TIC54X-Directives.
 
    Expansion is recursive until a previously encountered symbol is
 seen, at which point substitution stops.
 
    In this example, x is replaced with SYM2; SYM2 is replaced with
 SYM1, and SYM1 is replaced with x.  At this point, x has already been
 encountered and the substitution stops.
 
       .asg   "x",SYM1
       .asg   "SYM1",SYM2
       .asg   "SYM2",x
       add    x,a             ; final code assembled is "add  x, a"
 
    Macro parameters are converted to subsyms; a side effect of this is
 the normal `as' '\ARG' dereferencing syntax is unnecessary.  Subsyms
 defined within a macro will have global scope, unless the `.var'
 directive is used to identify the subsym as a local macro variable
  `.var' TIC54X-Directives.
 
    Substitution may be forced in situations where replacement might be
 ambiguous by placing colons on either side of the subsym.  The following
 code:
 
       .eval  "10",x
      LAB:X:  add     #x, a
 
    When assembled becomes:
 
      LAB10  add     #10, a
 
    Smaller parts of the string assigned to a subsym may be accessed with
 the following syntax:
 
 ``:SYMBOL(CHAR_INDEX):''
      Evaluates to a single-character string, the character at
      CHAR_INDEX.
 
 ``:SYMBOL(START,LENGTH):''
      Evaluates to a substring of SYMBOL beginning at START with length
      LENGTH.
 

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