| http://www.w3.org/ns/prov#value | - Referring to FIGS. 3-4, at least one of the sensors e.g. 11 comprises a switch means 11d mounted proximate a corresponding opening 6a, wherein the switch means 11d has: (i) an open position (FIG. 3) whereby the signal generator 16 is electrically disconnected from the power supply 18, and (ii) a closed position (FIG. 4) whereby the signal generator 16 is electrically connected to the power supply
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