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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
     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

   It is not an error for NAME to have no macro definition.  In that
case, `undefine' does nothing.

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