correct subset check + block verification optimization

faq.md

@@ -1,6 +1,6 @@
 ### Why is `Coroutine[Any, Any, T]` used instead of `Awaitable[T]`?
 * Typing issues (PyCharm)
-* Explicit expression of whether some sort of action is performed or not (tasks vs futures vs coros)
+* Explicit expression of whether some sort of action is performed or not (futures vs coros)
 
 ### Why is `subset-verify-optimized` branch closed?
 * Performance gain at large set sizes are 0% or less.

rainbowadn/flow13/_binaryflow.py

@@ -123,6 +123,7 @@ class SubCheck(
         if case.tree == self.split.tree:
             return SubTree()
         else:
+            assert case.split.key == self.key
             return Found()
 
     async def on_left(self, case: PBS) -> CheckResult:

rainbowadn/flow13/_flowblock.py

@@ -150,20 +150,7 @@ class FlowBlockIndexedVerification(
         return True
 
     async def _verify_previous(self, previous: HashPoint[FBL], block: FBL) -> bool:
-        async def verify_previous_contained() -> bool:
-            assert_true(await self.index.contains(previous))
-            return True
-
-        async def verify_previous_block() -> bool:
-            assert_true(await self._verify_previous_block(await previous.resolve(), block))
-            return True
-
-        assert_trues(
-            await gather(
-                verify_previous_block(),
-                verify_previous_contained(),
-            )
-        )
+        assert_true(await self._verify_previous_block(await previous.resolve(), block))
         return True
 
     async def _verify(self, block: FBL) -> bool: