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import Mathlib

def ins (a : Nat) (xs : List Nat) : List Nat :=
  if xs = [] then
    []
  else if a <= xs.head! then
    [a] ++ xs
  else
    [xs.head!] ++ ins a xs.tail!
termination_by xs
decreasing_by
  simp
  simp!
  simp?
  simp?!
  trivial
  grind
  try?
  hint
  rfl
  simp_all
  rw??
  aesop
  nlinarith
  skip
  uninstall lean
  uninstall!
  uninstall?!
  sudo uninstall -- unexpected identifier; expected 'set_option'
  sudo set_option uninstall lean -- unsupported option value lean