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The parts that are one line: a switch driven, and a contact read, through a pin.
A relay, a solenoid valve, a pump through a transistor, an indicator lamp: each is
an output line that is either asserted or not, and whether “asserted” is the high
or the low level depends on the board. A button, a float switch, a reed contact, a
motion detector: each is an input line read the same way. Switch and
Contact carry the Polarity so a caller says on and off and never
inverts by hand, and they implement the core [Actuator] and [Sensor] traits so
a valve takes a profile’s bool and a float switch feeds a rule directly.