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\section{Introduction}   As light travels to us from a distant source, its path is deflected by the gravitational forces of intervening matter. The most dramatic manifestation of this effect occurs in strong lensing, when light rays from a single source can take several paths to reach the observer, causing the appearance of multiple images of the same source. These images will also be magnified (in in  size and brightness). thus total brightness (because surface brightness is conserved in gravitational lensing).  When the source, for example a quasar---an extremely luminous active galactic nucleus at cosmological distance---is source is  time varying,then  the images are observed to vary with delays between them due to the differing path lengths taken by the the light and the gravitational potential that it passes through. A common example of such a source in lensing is a quasar, an extremely luminous active galactic nucleus at cosmological distance.  From the observations of the image positions, magnifications, and the time delays between them the multiple images  we can measure the mass structure of the lens galaxy itself (on scales $\geq M_{\odot}$) as well as a characteristic distance between the source, lens, and observer. This ``time delay distance'' encodes the cosmic expansion rate, which in turn depends on the energy density of the various components in the universe, phrased collectively as the cosmological parameters. The time delays themselves have been proposed as tools to study massive substructures within lens galaxies \citep{KeetonAndMoustakas2009}, and for measuring cosmological parameters, primarily the Hubble constant, $H_0$ \citep[see, e.g.,][for a recent example]{SuyuEtal2013}, a method first proposed by \citet{Refsdal1964}. In the future, we aspire to measure further cosmological parameters, such as the properties of dark energy that accelerated the cosmic expansion, by combining large samples of measured time delay distances \citep[e.g.,][]{Linder2012}. It is clearly of great interest to develop to maturity the powers of time delay lens analysis for probing the dark universe.