RE: on helm substantial differences

[Drew Adams]

> BTW, there is another TODO item in minibuffer.el:
> 
> ;;   - indicate how to display the completions in *Completions* (turn
> ;;     \n into something else, add special boundaries between
> ;;     completions).  E.g. when completing from the kill-ring.
> 
> Such pre-processing can be performed by the caller.

FWIW, Icicles does this.  If a completion candidate
is multi-line then it is separated from the following
candidate in *Completions* by a blank line (extra
newline).

Candidates are also distinguished in these ways:

* They have property `mouse-face'.  (Separator
  whitespace, including a newline, does not.)

* The "current" candidate (e.g. when cycling) is
  highlighted, which makes it and its limits stand
  out.