chore: add comments in erasable_pq.cpp

AGENTS.md

@@ -26,7 +26,7 @@ When adding or editing code here, optimize for contest usage first.
 - Prefer `ll` by default to reduce overflow debugging.
 - Use `int` only when clearly safe and beneficial (memory, bitset indexing, array indices, tight constraints).
 - Avoid implicit narrowing conversions. Cast explicitly at boundaries when mixing types.
-- Prefer aliases from `common.hpp` (e.g., `pll`, `pii`) when they match exactly to reduce typing.
+- Always use aliases from `common.hpp` (e.g., `pll`, `pii`) when they match exactly to reduce typing.
 
 ## Coding rules
 - Prefer straightforward implementations over heavy abstractions.

src/1-ds/erasable_pq.cpp

@@ -1,4 +1,9 @@
 #include "../common/common.hpp"
+// Erasable priority queue (lazy deletion).
+// what: priority_queue with erase by value (multiset-like)
+// time: push/pop/erase amortized O(log n), top O(1) amortized, memory: O(n)
+// constraint: erase only existing values, duplicates ok
+// usage: erasable_pq<ll> pq; pq.push(x); pq.erase(x); ll v = pq.top(); pq.pop();
 template <class T, class O = less<T>>
 struct pq_set {
     priority_queue<T, vector<T>, O> q, del;