This repository is a curated collection of Go packages extracted from the Go standard library's internal packages, plus a few CLI tools built on top of them. The primary package is `testscript`, a robust script-based testing framework that uses txtar archives as test scripts. It provides a declarative way to write integration tests with built-in commands (exec, cmp, stdout, stderr, etc.) and supports external plugins via JSON-RPC. Other notable packages include `diff` (an O(n log n) anchored diff algorithm), `txtar` (a simple human-readable archive format), `lockedfile` (cross-platform atomic file operations with locking), `robustio` (transparent retry for ephemeral I/O errors), `cache` (build artifact cache with SHA256 indexing), and `par` (parallel work executor with memoization). The repo also includes CLI commands: `cmd/testscript` (standalone runner), `cmd/txtar-c` (archive creation), `cmd/txtar-x` (extraction), `cmd/txtar-addmod` (add module to archive), and `cmd/txtar-goproxy` (serve modules from archives). The architecture is modular; each package is independent and can be reused separately. There are thin forwarding layers to `golang.org/x/mod` and `github.com/google/renameio` for compatibility. The codebase demonstrates high craftsmanship in package design, platform-specific abstractions (build tags), and testing utilities. The primary domain is Go tooling and testing infrastructure.