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#### 9.42.5 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 (See `.asg`, See `.eval`.

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 see `.var`.

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.