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The sequential behavior of CMOS logic circuits in the presence of stuck-open faults requires that an initialization input followed by a test input be applied to detect such a fault. However, a test set based on the assumption that delays through all gates and interconnections are zero, can be invalidated in the presence of arbitrary delays in the circuit. In this paper, we will present a necessary and sufficient condition for the existence of a test set, which cannot be invalidated under arbitrary delays, for an AND-OR or OR-AND CMOS realization for any given function. We will also introduce a Hybrid CMOS realization which, for any given function, is guaranteed to have a valid test set under arbitrary delays.