ring tactic fails on some inputs

[digama0]

Writing this down so I don't forget...

The ring tactic has some problem with finding normal forms here. A minimized example:

import tactic.ring
local infix `^` := monoid.pow
example {α} [comm_ring α] (x : α) (m n : ℕ) :
  x^2 + x^2 + x = x^2 + (x^2 + x) := by ring