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(m4.info)Pushdef

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Temporarily redefining macros
=============================

   It is possible to redefine a macro temporarily, reverting to the
previous definition at a later time.  This is done with the builtins
`pushdef' and `popdef':

     pushdef(NAME [, EXPANSION])
     popdef(NAME)

which are quite analogous to `define' and `undefine'.

   These macros work in a stack-like fashion.  A macro is temporarily
redefined with `pushdef', which replaces an existing definition of
NAME, while saving the previous definition, before the new one is
installed.  If there is no previous definition, `pushdef' behaves
exactly like `define'.

   If a macro has several definitions (of which only one is accessible),
the topmost definition can be removed with `popdef'.  If there is no
previous definition, `popdef' behaves like `undefine'.

     define(`foo', `Expansion one.')
     =>
     foo
     =>Expansion one.
     pushdef(`foo', `Expansion two.')
     =>
     foo
     =>Expansion two.
     popdef(`foo')
     =>
     foo
     =>Expansion one.
     popdef(`foo')
     =>
     foo
     =>foo

   If a macro with several definitions is redefined with `define', the
topmost definition is _replaced_ with the new definition.  If it is
removed with `undefine', _all_ the definitions are removed, and not
only the topmost one.

     define(`foo', `Expansion one.')
     =>
     foo
     =>Expansion one.
     pushdef(`foo', `Expansion two.')
     =>
     foo
     =>Expansion two.
     define(`foo', `Second expansion two.')
     =>
     foo
     =>Second expansion two.
     undefine(`foo')
     =>
     foo
     =>foo

   It is possible to temporarily redefine a builtin with `pushdef' and
`defn'.

   The macros `pushdef' and `popdef' are recognized only with
parameters.