docs(readme): minor edits

README.md

@@ -80,12 +80,14 @@ How-to:
 
 - **Count elements in an iterable:**: `applyPipe(yourIterable, reduceIterable(countReducer, 0))`.
 
-- **Check if every element in an iterable is true:** `applyPipe(yourIterableOfBooleans, reduceIterable(andReducer, true))` (iteration will not continue unnecessarily because of how [`reduceIterable`](https://github.com/ivan7237d/antiutils/blob/master/src/internal/iterable/reduceIterable.ts) and [`andReducer`](https://github.com/ivan7237d/antiutils/blob/master/src/internal/reducer/andReducer.ts) are defined).
+- **Check if every element in an iterable is true:** `applyPipe(yourIterableOfBooleans, reduceIterable(andReducer, true))` (iteration will not continue unnecessarily because of [how a reducer is defined](#reducers)).
 
 - **Check if some elements in an iterable are true:** `applyPipe(yourIterableOfBooleans, reduceIterable(orReducer, false))`.
 
 - **Find the first element matching a predicate:** `applyPipe(yourIterable, filter(yourPredicate), firstInIterable)`.
 
+- **Same as above, but throw an error if the iterable has multiple matching elements:** `applyPipe(yourIterable, filter(yourPredicate), reduceIterable(asContext(() => { throw yourError; })))` (`asContext` is described in the section ["Functions for downcasting"](#functions-for-downcasting)).
+
 - **Yield values while a condition holds:**
 
   ```ts