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import TestGame.Metadata
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import Mathlib
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Game "TestGame"
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World "SetFunction"
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Level 3
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Title ""
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Introduction
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"
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"
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Statement
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""
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(U : Set ℕ) (f : ℕ → ℕ) : U ⊆ f ⁻¹' (f '' U) := by
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intro x hx
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use x
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constructor
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assumption
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rfl
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