Idiomatic ways of selecting subterm to rewrite - coq

Question

Suppose we have a conclusion of form: a + b + c + d + e.
We also have a lemma: plus_assoc : forall n m p : nat, n + (m + p) = n + m + p.
What are idiomatic ways to arbitrarily "insert a pair of parentheses" into the term? That is, how can we easily choose where to rewrite if there's more than one available place.
What I generally end up doing is the following:
replace (a + b + c + d + e)
with (a + b + c + (d + e))
by now rewrite <- ?plus_assoc
And while this formulation does state exactly what I want to do,
it gets extremely long-winded for formulations more complicated than "a b c...".

Answer 1

rewrite <- lemma expects lemma to be an equality, that is, a term whose type is of the form something1 = something2. Like with most other tactics, you can also pass it a function that returns an equality, that is, a term whose type is of the form forall param1 … paramN, something1 = something2, in which case Coq will look for a place where it can apply the lemma to parameters to form a subterm of the goal. Coq's algorithm is deterministic, but letting it choose is not particularly useful except when performing repeated rewrites that eventually exhaust all possibilities. Here Coq happens to choose your desired goal with rewrite <- plus_assoc, but I assume that this was just an example and you're after a general technique.
You can get more control over where to perform the rewrite by supplying more parameters to the lemma, to get a more specific equality. For example, if you want to specify that (((a + b) + c) + d) + e should be turned into ((a + b) + c) + (d + e), i.e. that the associativity lemma should be applied to the parameters (a + b) + c, d and e, you can write
rewrite <- (plus_assoc ((a + b) + c) d e).
You don't need to supply all the parameters, just enough to pinpoint the place where you want to apply the lemma. For example, here, it's enough to specify d as the second argument. You can do this by leaving the third parameter out altogether and specifying the wildcard _ as the first parameter.
rewrite <- (plus_assoc _ d).
Occasionally there are identical subterms and you only want to rewrite one of them. In this case you can't use the rewrite family of tactics alone. One approach is to use replace with a bigger term where you pick what you want to change, or event assert to replace the whole goal. Another approach is to use the set tactics, which lets you give a name to a specific occurrence of a subterm, then rely on that name to identify specific subterms, and finally call subst to get rid of the name when you're done.
An alternative approach is to forget about which lemmas to apply, and just specify how you want to change the goal with something like assert or a plain replace … with ….. Then let automated tactics such as congruence, omega, solve [firstorder], etc. find parameters that make the proof work. With this approach, you do have to write down big parts of the goal, but you save on specifying lemmas. Which approach works best depends on where you are on a big proof and what tends to be stable during development and what isn't.

Answer 2

IMO your best option is to use the ssreflect pattern selection language, available in Coq 8.7 or by installing math-comp in earlier versions. This language is documented in the manual: https://hal.inria.fr/inria-00258384
Example (for Coq 8.7):
(* Replace with From mathcomp Require ... in Coq < 8.7 *)
From Coq Require Import ssreflect ssrfun ssrbool.
Lemma addnC n m : m + n = n + m. Admitted.
Lemma addnA m n o : m + (n + o) = m + n + o. Admitted.
Lemma example m n o p : n + o + p + m = m + n + o + p.
Proof. by rewrite -[_ + _ + o]addnA -[m + _ + p]addnA [m + _]addnC.
Qed.

Answer 3

If you don't want to prove a helper lemma, then one of your choices is using Ltac to pattern match on the structure of the equality on your hands. This way you can bind arbitrary complex subexpressions to pattern variables:
Require Import Coq.Arith.Arith.
Goal forall a b c d e,
(a + 1 + 2) + b + c + d + e = (a + 1 + 2) + (b + c + d) + e -> True.
intros a b c d e H.
match type of H with ?a + ?b + ?c + ?d + ?e = _ =>
replace (a + b + c + d + e)
with (a + (b + c + d) + e)
in H
by now rewrite <- ?plus_assoc
end.
Abort.
In the above piece of code ?a stands for a + 1 + 2. This, of course, doesn't improve anything if you are dealing with simple variables, it helps only when you are dealing with complex nested expressions.
Also, if you need to rewrite things in the goal, then you can use something like this:
match goal with
| |- ?a + ?b + ?c + ?d + ?e = _ => <call your tactics here>