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Time-delay cosmology is the use of the differences in arrival time of images that have undergone gravitational lensing to work out the geometry of the universe, e.g., between the lensing object and the EMR source. If the lensed source is transient, e.g., a supernova or active galactic nucleus (AGN), and a particular change detected in one lensed image is later detected in another, the delay offers information to help work out the geometry. The mass of the lens is useful for the analysis, thus is helpful if it can be determined.