Alist-keys-subsetp
Check that all keys of the alist belong to a given set
The call (alist-keys-subsetp alist keys) returns t when
each key of the given alist belongs to the given list of keys; else it returns
nil. This is Boolean-equivalent to (subsetp-eq (strip-cars alist)
keys), but it avoids consing up the keys of alist.