| http://www.w3.org/ns/prov#value | - The system of claim 13, wherein each source comprises a retroreflector which reflects at least two distinguishable colors, each color defining at least one state, and wherein the detector comprises a video camera, and further comprising:a source of light mounted substantially coaxially with the video camera; means for defining a plurality of cells scanned by the video camera, each cell including o
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