1258.lean:4:19: error: failed to revert 'xs', 'foo' depends on it, and 'foo' is an auxiliary declaration introduced by the equation compiler (possible solution: use tactic 'clear' to remove 'foo' from the local context)

state:
xs : list ℕ,
foo : ∀ (ns : list ℕ), P xs ns,
n : ℕ,
ns : list ℕ
⊢ P xs (n :: ns)