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m4: Undefine

 
 5.4 Deleting a macro
 ====================
 
 A macro definition can be removed with 'undefine':
 
  -- Builtin: undefine (NAME...)
      For each argument, remove the macro NAME.  The macro names must
      necessarily be quoted, since they will be expanded otherwise.
 
      The expansion of 'undefine' is void.  The macro 'undefine' is
      recognized only with parameters.
 
      foo bar blah
      =>foo bar blah
      define(`foo', `some')define(`bar', `other')define(`blah', `text')
      =>
      foo bar blah
      =>some other text
      undefine(`foo')
      =>
      foo bar blah
      =>foo other text
      undefine(`bar', `blah')
      =>
      foo bar blah
      =>foo bar blah
 
    Undefining a macro inside that macro's expansion is safe; the macro
 still expands to the definition that was in effect at the '('.
 
      define(`f', ``$0':$1')
      =>
      f(f(f(undefine(`f')`hello world')))
      =>f:f:f:hello world
      f(`bye')
      =>f(bye)
 
    It is not an error for NAME to have no macro definition.  In that
 case, 'undefine' does nothing.
 

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