-* Arrays/slices
-* Trait objects
-* Arc, concurrency, channels: Some grep-like thing, "rgrep"
-* Send, Sync
-* External dependencies: regexp crate, add to rgrep
-
-* Shared-memoty concurrency, interior mutability: Concurrent counter
+* Shared-memoty concurrency, interior mutability, Sync: Concurrent counter
* Drop, unsafe: doubly-linked list