# Documentation

Mathlib.Tactic.RestateAxiom

DEPRECATED: restate_axiom was necessary in Lean 3 but is no longer needed for Lean 4. It is still present for backwards compatibility but will probably be removed in the future.

# Original Docstring #

restate_axiom makes a new copy of a structure field, first definitionally simplifying the type. This is useful to remove auto_param or opt_param from the statement.

As an example, we have:

structure A :=
(x : ℕ)
(a' : x = 1 . skip)

example (z : A) : z.x = 1 := by rw A.a' -- rewrite tactic failed, lemma is not an equality nor a iff

restate_axiom A.a'
example (z : A) : z.x = 1 := by rw A.a


By default, restate_axiom names the new lemma by removing a trailing ', or otherwise appending _lemma if there is no trailing '. You can also give restate_axiom a second argument to specify the new name, as in

restate_axiom A.a f
example (z : A) : z.x = 1 := by rw A.f

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