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(nasm.info)Section 4.1.4

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4.1.4. Undefining macros: `%undef'
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   Single-line macros can be removed with the `%undef' command. For
example, the following sequence:

     %define foo bar
     %undef  foo
     
             mov     eax, foo

   will expand to the instruction `mov eax, foo', since after `%undef'
the macro `foo' is no longer defined.

   Macros that would otherwise be pre-defined can be undefined on the
command- line using the `-u' option on the NASM command line: see *Note
Section 2.1.13::.