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KEY: Impenetrable, [See PENETRABLE].
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KEY: Impenitent, [See PENITENT].
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KEY: Imperative.
SYN: Urgent, irresistible, dictatorial, inexorable, peremptorily, compulsory, obligatory.
ANT: Indulgent, lenient, mild, entreative, supplicatory, optional, discretional.
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KEY: Imperceptible, [See PERCEPTIBLE].