517899da7b
This PR implements tactics called `extract_lets` and `lift_lets` that manipulate `let`/`let_fun` expressions. The `extract_lets` tactic creates new local declarations extracted from any `let` and `let_fun` expressions in the main goal. For top-level lets in the target, it is like the `intros` tactic, but in general it can extract lets from deeper subexpressions as well. The `lift_lets` tactic moves `let` and `let_fun` expressions as far out of an expression as possible, but it does not extract any new local declarations. The option `extract_lets +lift` combines these behaviors. This is a re-implementation of `extract_lets` and `lift_lets` from mathlib. The new `extract_lets` is like doing `lift_lets; extract_lets`, but it does not lift unextractable lets like `lift_lets`. The `lift_lets; extract_lets` behavior is now handled by `extract_lets +lift`. The new `lift_lets` tactic is a frontend to `extract_lets +lift` machinery, which rather than creating new local definitions instead represents the accumulated local declarations as top-level lets. There are also conv tactics for both of these. The `extract_lets` has a limitation due to the conv architecture; it can extract lets for a given conv goal, but the local declarations don't survive outside conv. They get zeta reduced immediately upon leaving conv. |
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