Variable: minibuffer-default-add-function

Function run by `goto-history-element' before consuming default values.
This is useful to dynamically add more elements to the list of default values
when `goto-history-element' reaches the end of this list.
Before calling this function `goto-history-element' sets the variable
`minibuffer-default-add-done' to t, so it will call this function only
once. In special cases, when this function needs to be called more
than once, it can set `minibuffer-default-add-done' to nil explicitly,
overriding the setting of this variable to t in `goto-history-element'.