impedance matching impedance measurement impedance measurements impedance must impedance value impedance values impedances impede impede progress impeded impedes impediment impedimenta impediments impeding impeding a federal impeding the implementation impel impel down impelled impeller impeller blade impeller blades impeller shaft impeller wheel impellers impels impels us impending impending apocalypse impending arrival impending attack impending catastrophe impending crisis impending danger impending death impending destruction impending disaster impending doom impending expiry impending invasion impending judgment impending marriage impending war impenetrable impenetrable barrier impenetrable forest impenetrable fortress impenetrable wall impenitent imperative imperative form imperative i imperative mood imperative need imperative programming imperative that you imperative to have imperative to make imperative to understand imperative you imperatives imperator imperceptible imperceptibly imperfect imperfect competition imperfect human imperfect humans imperfect information imperfect people imperfect person imperfect science imperfect world imperfection imperfections imperfections can imperfections make imperfective imperfectly imperfectly perfect imperforate imperia imperia online imperial impérial imperial academy imperial academy of arts imperial administration imperial age imperial airways imperial ambitions imperial and royal imperial arms imperial army imperial atlantic imperial austrian imperial authorities imperial authority