353.lean:13:27-13:35: error: typeclass instance problem is stuck
  Arr.{1, ?u} Bool (@?m a₁ a₂)

Note: Lean will not try to resolve this typeclass instance problem because the second type argument to `Arr.{u1,
  u2}` contains metavariables. This argument must be fully determined before Lean will try to resolve the typeclass.

Hint: Adding type annotations and supplying implicit arguments to functions can give Lean more information for typeclass resolution. For example, if you have a variable `x` that you intend to be a `Nat`, but Lean reports it as having an unresolved type like `?m`, replacing `x` with `(x : Nat)` can get typeclass resolution un-stuck.