Use `map-indexed` when a transformation needs both a collection element and its position. With a collection, `(map-indexed f coll)` returns a lazy sequence; `f` receives the zero-based index first and the element second. section 3.5.2
Make integer-keyed entries
`(into {} (map-indexed vector [:a :b :c]))` returns `{0 :a, 1 :b, 2 :c}`. Each call to `vector` pairs an index with its element; `into` collects those pairs as a map. section 3.5.2
Number displayed results
`(map-indexed (fn [idx ticket] (format "winner %s: %s" (inc idx) ticket)) ["QA123A3" "ZR2345Z"])` produces `("winner 1: QA123A3" "winner 2: ZR2345Z")`. Increment the index when the display should start at one. section 3.5.2
Unlike `map`, which can take multiple collections, `map-indexed` accepts one collection or just a function. `(map-indexed f)` returns a transducer for use in a transformation pipeline. Because sequence results are lazy, avoid relying on an exact once-only schedule for side effects in `f`. section 3.5.2 section 7.2
