simp [set]
n : ℕ
pos' pos : Pos n
p? : Option PPiece
b : Board n
h : pos' ≠ pos
⊢ set b pos p? pos' = b pos'
n : ℕ
pos' pos : Pos n
p? : Option PPiece
b : Board n
h : pos' ≠ pos
⊢ pos = pos' → p? = b pos'