OPTIONAL
SYNOPSIS
        mixed * intersect_alist (mixed * list1, mixed * list2)

DESCRIPTION
        Does a fast set intersection on alist key vectors (NOT on full alists!).
        The operator '&' does set intersection on arrays in general.

EXAMPLE
        new_list = intersect_alist(list1, list2);

SEE ALSO
        filter(E), assoc(E), insert_alist(E), map(E),
        member_array(E), order_alist(E), sort_array(E), unique_array(E)