RE: cl-delete-if vs list of overlays

[Drew Adams]

> I was under the (wrong) assumption that a destructive function will
> always modify the list, when it's not the case.  I needed to setq the
> result of cl-delete-if to the list itself to cover all the cases.

Yes.  When you pass a list-valued variable to a "destructive"
function you pretty much always want to set the variable to
the result.

`C-h i' Elisp manual `i destructive list operations'

And `g Sets and Lists' tells us, about using `delq':

  Don’t assume that a variable which formerly held the
  argument LIST now has fewer elements, or that it still
  holds the original list!  Instead, save the result of
  ‘delq’ and use that.  Most often we store the result
  back into the variable that held the original list: