> +(defcustom imenu-flatten nil
> +  "If non-nil, popup the completion buffer with a flattened menu.
> +The string from `imenu-level-separator' is used to separate names of
> +nested levels while flattening nested indexes with name concatenation."
> +  :type 'boolean

Here is another useful value for `imenu-flatten' that
will show section names as a suffix instead of a prefix: