Alist of persistent window parameters.
This alist specifies which window parameters shall get saved by
`current-window-configuration' and `window-state-get' and subsequently
restored to their previous values by `set-window-configuration' and
`window-state-put'.
The car of each entry of this alist is the symbol specifying the
parameter. The cdr is one of the following:
nil means the parameter is neither saved by `window-state-get' nor by
`current-window-configuration'.
t means the parameter is saved by `current-window-configuration' and,
provided its WRITABLE argument is nil, by `window-state-get'.
The symbol `writable' means the parameter is saved unconditionally by
both `current-window-configuration' and `window-state-get'. Do not use
this value for parameters without read syntax (like windows or frames).
Parameters not saved by `current-window-configuration' or
`window-state-get' are left alone by `set-window-configuration'
respectively are not installed by `window-state-put'.