A fundamental problem with defcustoms that are lists

[Lennart Borgman (gmail)]

If you have a defcustom which uses variable length lists

  (defcustom my-option
    :type '(repeat ...)
    )

then you may want the default values from the list but also add some of
your own.

If you customize such an option and the default values are changed you
will not notice because when your setting is used Emacs never sees the
changes in default.

This mean that any enhancements or bug corrections to the default will
never be used.

Wouldn't it be worthwhile to give some way to add entries at the
begining or end of the list but add those to the default value instead
of just replacing the default value?