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I haven't reread "Monday" for a long time, unfortunately, but I really enjoyed the story about the homunculus Vybeghallo and the Great Seal.
But if I'm not mistaken, the Great Seal was in "The Tale of Three."
My respect! The Strugatsky brothers are great! Especially "Monday," "Picnic on the Roadside," "The Dead Mountaineer's Hotel," "The Snails on the Slope," and "Child."
And we should also remember "The Inhabited Island," "Beetle in the Anthill," "Waves Extinguish the Wind," "Far Rainbow," "The City Doomed," "Predatory Things of the Century," "Return. Noon. XXII Century," and so on. In general, you can read anything by the Strugatsky brothers; you won't go wrong.