feat: Corners, corner-free sets

[YaelDillies]

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For G a commutative monoid and a set A in G × G , define corners of A as solutions to x₁ + y₂ = x₂ + y₁ where (x₁, y₁) ∈ A, (x₁, y₂) ∈ A, (x₂, y₁) ∈ A.

Define corner-free sets as sets containing no non-trivial corners.

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