change-children

Makes arbitrary changes to the panel's list of children and updates the display.

• (send a-mred:single-panel change-children f) -> void
      f : a function which takes a list of children and returns a list of children.

  Examines the result of applying f to the current list of children. If some members of this list have parents other than this panel, issues an error. Otherwise, checks to ensure that the active child has not been deleted. If the active child has been deleted, resets it to null. Sets the panel's list of children to the result of f and forces a redraw.