I've noticed what appear to be inconsistent labelling on some list
functions, and I'm wondering if I'm properly understanding the reasons
behind the way the labels work.

For example, in the various association list functions, in some cases
the association list is passed with a ~map label, and sometimes with
no label.  Another odd case is the mem and memq functions, both of
which label the list being queried with the label ~set.  In this case,
the labelling mostly seems kind of useless rather than inconsistent.

I'm asking all of this because I'm playing around with writing a
labelled version of the extlib interface, and I'm wondering whether
these are mistakes that should be fixed, or whether there are good
reasons for them and they should be preserved.

Yaron