A photon source capable of emitting, for example, a single photon or a single pair of photons on demand. A method may comprise contacting an acyliminepyridine compound, a metal salt, and an amine to form a mixture; and recovering the hexadentate bimetallic complex from the mixture. The apparatus for maintenance of a ship propeller under water comprises a flexible habitat with an open upper end. At least two modified representations are then identified as possible representations of syntactic structures in the sentence. The scraping arrangement includes blades that are driven along a joint or crack by the cart to remove excess filling material deposited in the joint or crack of a working surface. The scaled data is modulated and the resulting modulated, scaled data is converted from digital to analog form.