# The `norm_cast`

family of tactics. #

A full description of the tactic, and the use of each theorem category, can be found at

Proves `a = b`

using the given simp set.

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Proves `a = b`

by simplifying using move and squash lemmas.

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Constructs the expression `(e : ty)`

.

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- Lean.Elab.Tactic.NormCast.mkCoe e ty = do let __discr ← Lean.Meta.coerce? e ty match __discr with | Lean.LOption.some e' => pure e' | x => failure

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Checks whether an expression is the coercion of some other expression, and if so returns that expression.

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This is the main heuristic used alongside the elim and move lemmas. The goal is to help casts move past operators by adding intermediate casts. An expression of the shape:

```
op (↑(x : α) : γ) (↑(y : β) : γ)
```

is rewritten to:

```
op (↑(↑(x : α) : β) : γ) (↑(y : β) : γ)
```

when

```
(↑(↑(x : α) : β) : γ) = (↑(x : α) : γ)
```

can be proven with a squash lemma

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Discharging function used during simplification in the "squash" step.

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Core rewriting function used in the "squash" step, which moves casts upwards and eliminates them.

It tries to rewrite an expression using the elim and move lemmas. On failure, it calls the splitting procedure heuristic.

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If possible, rewrites `(n : α)`

to `(Nat.cast n : α)`

where `n`

is a numeral and `α ≠ ℕ`

.
Returns a pair of the new expression and proof that they are equal.

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The core simplification routine of `normCast`

.

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Implementation of the `norm_cast`

tactic when operating on the main goal.

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Implementation of the `norm_cast`

tactic when operating on a hypothesis.

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