Builtin Commands

  [Memory]        cmd/append             Add elements to end of array
                  pp                     value   proc      test_
                                         asdl_   cell_   X gc-stats_
  [Handle Errors] error                  error 'failed' (status=2)
                  try                    Run with errexit, set _error
                  failed                 Test if _error.code !== 0
                  boolstatus             Enforce 0 or 1 exit status
                  assert                 assert [42 === f(x)]
  [Shell State]   ysh-cd       ysh-shopt compatible, and takes a block
                  shvar                  Temporary modify global settings
                  ctx                    Share and update a temporary "context"
                  push-registers         Save registers like $?, PIPESTATUS
  [Word Lookup]   invoke                 Run a command, and control name lookup
                  runproc                Run a proc; use as main entry point
                X extern                 Run an external command, with an ENV
  [Modules]       
                  source-guard           guard against duplicate 'source'
                  is-main                false when sourcing a file
                  use                    create a module Obj from a source file
  [I/O]           ysh-read               flags --all, -0
                  ysh-echo               no -e -n with simple_echo
                  ysh-test               --file --true etc.
                  ysh-wait               wait --all --verbose
                  write                  Like echo, with --, --sep, --end
                  fork         forkwait  Replace & and (), and takes a block
                  fopen                  Open multiple streams, takes a block
  [Private]       cat          rm        POSIX-compatible
                  sleep
  [Hay Config]    hay          haynode   For DSLs and config files
  [Completion]    compadjust   compexport
  [Data Formats]  json                   read write
                  json8                  read write
