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Close App A when App B closes: Mac OS X 10.7.3
if not exists (processes where name is A)
   tell application B to quit
end if

set theNotice to current application's NSWorkspaceDidTerminateApplicationNotification
    tell current application's NSWorkspace's sharedWorkspace's notificationCenter
        addObserver_selector_name_object_(me, "appQuitNotification:", theNotice, missing value)
    end tell

on appQuitNotification_(aNotification) -- an application quit
    -- aNotification's userInfo record contains an NSRunningApplication instance that we can get properties from
    set theApplication to (aNotification's userInfo's NSWorkspaceApplicationKey's localizedName()) as text
    say "application " & theApplication & " quit" -- for testing
    if theApplication is "this" then tell application "that" to quit -- or whatever
end appQuitNotification_