Fixpoint keyAssignmentsRemoved
  (ka : list keyAssignment)
  (kb : list keyAssignment)
: list keyAssignment :=
  match ka with
  | nil => []
  | cons a rest =>
    match keyAssignmentForId (kaId a) kb with
    | None   => cons a (keyAssignmentsRemoved rest kb)
    | Some _ => keyAssignmentsRemoved rest kb
    end
  end.
