impregnable, invulnerable, impenetrable, inviolable, invincible, unconquerable, unattackable
indisputable, undeniable, unquestionable, incontestable, incontrovertible, irrefutable, indubitable, watertight, sound, conclusive, absolute, positive, proven, beyond dispute, beyond question, beyond doubt, beyond a shadow of a doubt, certain, sure, manifest, patent, obvious
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