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This paper is concerned with the problem of designing easily testable sequential machines, output-observable machines, for which there exist very short checking experiments. A sequential machine for which any initial state can be uniquely determined only by the output response is said to be output-observable. An algorithm is developed to modify a given machine to an output-observable one by adding a minimum number of extra outputs. This method is based on the fact that the output-observable realization of a given machine M exists if and only if M is semi-FSR realizable (a special type of feedback shift register realization).