Consider:
  File | New | Census
  Census | Run selected cell
  close the census window that's now in the background
The illustration window that remains open is subordinate to the
closed census window; lmi ought to close it automatically, but
doesn't. This is puzzling to end users, who by design can't edit
the properties of the individual illustration once the census is
closed. It is quite bothersome when many cells in a census have
been run: each must be closed manually.

The defunct 'owl' library addressed this with its 'child document'
concept. There being little demand for that feature in wx, a
solution within lmi should be considered.