Variable: imenu-prev-index-position-function

Function for finding the next index position.

If `imenu-create-index-function' is set to
`imenu-default-create-index-function', then you must set this variable
to a function that will find the next index, looking backwards in the

The function should leave point at the place to be connected to the
index and it should return nil when it doesn't find another index.